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diff --git a/contrib/tools/python3/Lib/email/_header_value_parser.py b/contrib/tools/python3/Lib/email/_header_value_parser.py
new file mode 100644
index 0000000000..5b653f66c1
--- /dev/null
+++ b/contrib/tools/python3/Lib/email/_header_value_parser.py
@@ -0,0 +1,3008 @@
+"""Header value parser implementing various email-related RFC parsing rules.
+
+The parsing methods defined in this module implement various email related
+parsing rules.  Principal among them is RFC 5322, which is the followon
+to RFC 2822 and primarily a clarification of the former.  It also implements
+RFC 2047 encoded word decoding.
+
+RFC 5322 goes to considerable trouble to maintain backward compatibility with
+RFC 822 in the parse phase, while cleaning up the structure on the generation
+phase.  This parser supports correct RFC 5322 generation by tagging white space
+as folding white space only when folding is allowed in the non-obsolete rule
+sets.  Actually, the parser is even more generous when accepting input than RFC
+5322 mandates, following the spirit of Postel's Law, which RFC 5322 encourages.
+Where possible deviations from the standard are annotated on the 'defects'
+attribute of tokens that deviate.
+
+The general structure of the parser follows RFC 5322, and uses its terminology
+where there is a direct correspondence.  Where the implementation requires a
+somewhat different structure than that used by the formal grammar, new terms
+that mimic the closest existing terms are used.  Thus, it really helps to have
+a copy of RFC 5322 handy when studying this code.
+
+Input to the parser is a string that has already been unfolded according to
+RFC 5322 rules.  According to the RFC this unfolding is the very first step, and
+this parser leaves the unfolding step to a higher level message parser, which
+will have already detected the line breaks that need unfolding while
+determining the beginning and end of each header.
+
+The output of the parser is a TokenList object, which is a list subclass.  A
+TokenList is a recursive data structure.  The terminal nodes of the structure
+are Terminal objects, which are subclasses of str.  These do not correspond
+directly to terminal objects in the formal grammar, but are instead more
+practical higher level combinations of true terminals.
+
+All TokenList and Terminal objects have a 'value' attribute, which produces the
+semantically meaningful value of that part of the parse subtree.  The value of
+all whitespace tokens (no matter how many sub-tokens they may contain) is a
+single space, as per the RFC rules.  This includes 'CFWS', which is herein
+included in the general class of whitespace tokens.  There is one exception to
+the rule that whitespace tokens are collapsed into single spaces in values: in
+the value of a 'bare-quoted-string' (a quoted-string with no leading or
+trailing whitespace), any whitespace that appeared between the quotation marks
+is preserved in the returned value.  Note that in all Terminal strings quoted
+pairs are turned into their unquoted values.
+
+All TokenList and Terminal objects also have a string value, which attempts to
+be a "canonical" representation of the RFC-compliant form of the substring that
+produced the parsed subtree, including minimal use of quoted pair quoting.
+Whitespace runs are not collapsed.
+
+Comment tokens also have a 'content' attribute providing the string found
+between the parens (including any nested comments) with whitespace preserved.
+
+All TokenList and Terminal objects have a 'defects' attribute which is a
+possibly empty list all of the defects found while creating the token.  Defects
+may appear on any token in the tree, and a composite list of all defects in the
+subtree is available through the 'all_defects' attribute of any node.  (For
+Terminal notes x.defects == x.all_defects.)
+
+Each object in a parse tree is called a 'token', and each has a 'token_type'
+attribute that gives the name from the RFC 5322 grammar that it represents.
+Not all RFC 5322 nodes are produced, and there is one non-RFC 5322 node that
+may be produced: 'ptext'.  A 'ptext' is a string of printable ascii characters.
+It is returned in place of lists of (ctext/quoted-pair) and
+(qtext/quoted-pair).
+
+XXX: provide complete list of token types.
+"""
+
+import re
+import sys
+import urllib   # For urllib.parse.unquote
+from string import hexdigits
+from operator import itemgetter
+from email import _encoded_words as _ew
+from email import errors
+from email import utils
+
+#
+# Useful constants and functions
+#
+
+WSP = set(' \t')
+CFWS_LEADER = WSP | set('(')
+SPECIALS = set(r'()<>@,:;.\"[]')
+ATOM_ENDS = SPECIALS | WSP
+DOT_ATOM_ENDS = ATOM_ENDS - set('.')
+# '.', '"', and '(' do not end phrases in order to support obs-phrase
+PHRASE_ENDS = SPECIALS - set('."(')
+TSPECIALS = (SPECIALS | set('/?=')) - set('.')
+TOKEN_ENDS = TSPECIALS | WSP
+ASPECIALS = TSPECIALS | set("*'%")
+ATTRIBUTE_ENDS = ASPECIALS | WSP
+EXTENDED_ATTRIBUTE_ENDS = ATTRIBUTE_ENDS - set('%')
+
+def quote_string(value):
+    return '"'+str(value).replace('\\', '\\\\').replace('"', r'\"')+'"'
+
+# Match a RFC 2047 word, looks like =?utf-8?q?someword?=
+rfc2047_matcher = re.compile(r'''
+   =\?            # literal =?
+   [^?]*          # charset
+   \?             # literal ?
+   [qQbB]         # literal 'q' or 'b', case insensitive
+   \?             # literal ?
+  .*?             # encoded word
+  \?=             # literal ?=
+''', re.VERBOSE | re.MULTILINE)
+
+
+#
+# TokenList and its subclasses
+#
+
+class TokenList(list):
+
+    token_type = None
+    syntactic_break = True
+    ew_combine_allowed = True
+
+    def __init__(self, *args, **kw):
+        super().__init__(*args, **kw)
+        self.defects = []
+
+    def __str__(self):
+        return ''.join(str(x) for x in self)
+
+    def __repr__(self):
+        return '{}({})'.format(self.__class__.__name__,
+                             super().__repr__())
+
+    @property
+    def value(self):
+        return ''.join(x.value for x in self if x.value)
+
+    @property
+    def all_defects(self):
+        return sum((x.all_defects for x in self), self.defects)
+
+    def startswith_fws(self):
+        return self[0].startswith_fws()
+
+    @property
+    def as_ew_allowed(self):
+        """True if all top level tokens of this part may be RFC2047 encoded."""
+        return all(part.as_ew_allowed for part in self)
+
+    @property
+    def comments(self):
+        comments = []
+        for token in self:
+            comments.extend(token.comments)
+        return comments
+
+    def fold(self, *, policy):
+        return _refold_parse_tree(self, policy=policy)
+
+    def pprint(self, indent=''):
+        print(self.ppstr(indent=indent))
+
+    def ppstr(self, indent=''):
+        return '\n'.join(self._pp(indent=indent))
+
+    def _pp(self, indent=''):
+        yield '{}{}/{}('.format(
+            indent,
+            self.__class__.__name__,
+            self.token_type)
+        for token in self:
+            if not hasattr(token, '_pp'):
+                yield (indent + '    !! invalid element in token '
+                                        'list: {!r}'.format(token))
+            else:
+                yield from token._pp(indent+'    ')
+        if self.defects:
+            extra = ' Defects: {}'.format(self.defects)
+        else:
+            extra = ''
+        yield '{}){}'.format(indent, extra)
+
+
+class WhiteSpaceTokenList(TokenList):
+
+    @property
+    def value(self):
+        return ' '
+
+    @property
+    def comments(self):
+        return [x.content for x in self if x.token_type=='comment']
+
+
+class UnstructuredTokenList(TokenList):
+    token_type = 'unstructured'
+
+
+class Phrase(TokenList):
+    token_type = 'phrase'
+
+class Word(TokenList):
+    token_type = 'word'
+
+
+class CFWSList(WhiteSpaceTokenList):
+    token_type = 'cfws'
+
+
+class Atom(TokenList):
+    token_type = 'atom'
+
+
+class Token(TokenList):
+    token_type = 'token'
+    encode_as_ew = False
+
+
+class EncodedWord(TokenList):
+    token_type = 'encoded-word'
+    cte = None
+    charset = None
+    lang = None
+
+
+class QuotedString(TokenList):
+
+    token_type = 'quoted-string'
+
+    @property
+    def content(self):
+        for x in self:
+            if x.token_type == 'bare-quoted-string':
+                return x.value
+
+    @property
+    def quoted_value(self):
+        res = []
+        for x in self:
+            if x.token_type == 'bare-quoted-string':
+                res.append(str(x))
+            else:
+                res.append(x.value)
+        return ''.join(res)
+
+    @property
+    def stripped_value(self):
+        for token in self:
+            if token.token_type == 'bare-quoted-string':
+                return token.value
+
+
+class BareQuotedString(QuotedString):
+
+    token_type = 'bare-quoted-string'
+
+    def __str__(self):
+        return quote_string(''.join(str(x) for x in self))
+
+    @property
+    def value(self):
+        return ''.join(str(x) for x in self)
+
+
+class Comment(WhiteSpaceTokenList):
+
+    token_type = 'comment'
+
+    def __str__(self):
+        return ''.join(sum([
+                            ["("],
+                            [self.quote(x) for x in self],
+                            [")"],
+                            ], []))
+
+    def quote(self, value):
+        if value.token_type == 'comment':
+            return str(value)
+        return str(value).replace('\\', '\\\\').replace(
+                                  '(', r'\(').replace(
+                                  ')', r'\)')
+
+    @property
+    def content(self):
+        return ''.join(str(x) for x in self)
+
+    @property
+    def comments(self):
+        return [self.content]
+
+class AddressList(TokenList):
+
+    token_type = 'address-list'
+
+    @property
+    def addresses(self):
+        return [x for x in self if x.token_type=='address']
+
+    @property
+    def mailboxes(self):
+        return sum((x.mailboxes
+                    for x in self if x.token_type=='address'), [])
+
+    @property
+    def all_mailboxes(self):
+        return sum((x.all_mailboxes
+                    for x in self if x.token_type=='address'), [])
+
+
+class Address(TokenList):
+
+    token_type = 'address'
+
+    @property
+    def display_name(self):
+        if self[0].token_type == 'group':
+            return self[0].display_name
+
+    @property
+    def mailboxes(self):
+        if self[0].token_type == 'mailbox':
+            return [self[0]]
+        elif self[0].token_type == 'invalid-mailbox':
+            return []
+        return self[0].mailboxes
+
+    @property
+    def all_mailboxes(self):
+        if self[0].token_type == 'mailbox':
+            return [self[0]]
+        elif self[0].token_type == 'invalid-mailbox':
+            return [self[0]]
+        return self[0].all_mailboxes
+
+class MailboxList(TokenList):
+
+    token_type = 'mailbox-list'
+
+    @property
+    def mailboxes(self):
+        return [x for x in self if x.token_type=='mailbox']
+
+    @property
+    def all_mailboxes(self):
+        return [x for x in self
+            if x.token_type in ('mailbox', 'invalid-mailbox')]
+
+
+class GroupList(TokenList):
+
+    token_type = 'group-list'
+
+    @property
+    def mailboxes(self):
+        if not self or self[0].token_type != 'mailbox-list':
+            return []
+        return self[0].mailboxes
+
+    @property
+    def all_mailboxes(self):
+        if not self or self[0].token_type != 'mailbox-list':
+            return []
+        return self[0].all_mailboxes
+
+
+class Group(TokenList):
+
+    token_type = "group"
+
+    @property
+    def mailboxes(self):
+        if self[2].token_type != 'group-list':
+            return []
+        return self[2].mailboxes
+
+    @property
+    def all_mailboxes(self):
+        if self[2].token_type != 'group-list':
+            return []
+        return self[2].all_mailboxes
+
+    @property
+    def display_name(self):
+        return self[0].display_name
+
+
+class NameAddr(TokenList):
+
+    token_type = 'name-addr'
+
+    @property
+    def display_name(self):
+        if len(self) == 1:
+            return None
+        return self[0].display_name
+
+    @property
+    def local_part(self):
+        return self[-1].local_part
+
+    @property
+    def domain(self):
+        return self[-1].domain
+
+    @property
+    def route(self):
+        return self[-1].route
+
+    @property
+    def addr_spec(self):
+        return self[-1].addr_spec
+
+
+class AngleAddr(TokenList):
+
+    token_type = 'angle-addr'
+
+    @property
+    def local_part(self):
+        for x in self:
+            if x.token_type == 'addr-spec':
+                return x.local_part
+
+    @property
+    def domain(self):
+        for x in self:
+            if x.token_type == 'addr-spec':
+                return x.domain
+
+    @property
+    def route(self):
+        for x in self:
+            if x.token_type == 'obs-route':
+                return x.domains
+
+    @property
+    def addr_spec(self):
+        for x in self:
+            if x.token_type == 'addr-spec':
+                if x.local_part:
+                    return x.addr_spec
+                else:
+                    return quote_string(x.local_part) + x.addr_spec
+        else:
+            return '<>'
+
+
+class ObsRoute(TokenList):
+
+    token_type = 'obs-route'
+
+    @property
+    def domains(self):
+        return [x.domain for x in self if x.token_type == 'domain']
+
+
+class Mailbox(TokenList):
+
+    token_type = 'mailbox'
+
+    @property
+    def display_name(self):
+        if self[0].token_type == 'name-addr':
+            return self[0].display_name
+
+    @property
+    def local_part(self):
+        return self[0].local_part
+
+    @property
+    def domain(self):
+        return self[0].domain
+
+    @property
+    def route(self):
+        if self[0].token_type == 'name-addr':
+            return self[0].route
+
+    @property
+    def addr_spec(self):
+        return self[0].addr_spec
+
+
+class InvalidMailbox(TokenList):
+
+    token_type = 'invalid-mailbox'
+
+    @property
+    def display_name(self):
+        return None
+
+    local_part = domain = route = addr_spec = display_name
+
+
+class Domain(TokenList):
+
+    token_type = 'domain'
+    as_ew_allowed = False
+
+    @property
+    def domain(self):
+        return ''.join(super().value.split())
+
+
+class DotAtom(TokenList):
+    token_type = 'dot-atom'
+
+
+class DotAtomText(TokenList):
+    token_type = 'dot-atom-text'
+    as_ew_allowed = True
+
+
+class NoFoldLiteral(TokenList):
+    token_type = 'no-fold-literal'
+    as_ew_allowed = False
+
+
+class AddrSpec(TokenList):
+
+    token_type = 'addr-spec'
+    as_ew_allowed = False
+
+    @property
+    def local_part(self):
+        return self[0].local_part
+
+    @property
+    def domain(self):
+        if len(self) < 3:
+            return None
+        return self[-1].domain
+
+    @property
+    def value(self):
+        if len(self) < 3:
+            return self[0].value
+        return self[0].value.rstrip()+self[1].value+self[2].value.lstrip()
+
+    @property
+    def addr_spec(self):
+        nameset = set(self.local_part)
+        if len(nameset) > len(nameset-DOT_ATOM_ENDS):
+            lp = quote_string(self.local_part)
+        else:
+            lp = self.local_part
+        if self.domain is not None:
+            return lp + '@' + self.domain
+        return lp
+
+
+class ObsLocalPart(TokenList):
+
+    token_type = 'obs-local-part'
+    as_ew_allowed = False
+
+
+class DisplayName(Phrase):
+
+    token_type = 'display-name'
+    ew_combine_allowed = False
+
+    @property
+    def display_name(self):
+        res = TokenList(self)
+        if len(res) == 0:
+            return res.value
+        if res[0].token_type == 'cfws':
+            res.pop(0)
+        else:
+            if res[0][0].token_type == 'cfws':
+                res[0] = TokenList(res[0][1:])
+        if res[-1].token_type == 'cfws':
+            res.pop()
+        else:
+            if res[-1][-1].token_type == 'cfws':
+                res[-1] = TokenList(res[-1][:-1])
+        return res.value
+
+    @property
+    def value(self):
+        quote = False
+        if self.defects:
+            quote = True
+        else:
+            for x in self:
+                if x.token_type == 'quoted-string':
+                    quote = True
+        if len(self) != 0 and quote:
+            pre = post = ''
+            if self[0].token_type=='cfws' or self[0][0].token_type=='cfws':
+                pre = ' '
+            if self[-1].token_type=='cfws' or self[-1][-1].token_type=='cfws':
+                post = ' '
+            return pre+quote_string(self.display_name)+post
+        else:
+            return super().value
+
+
+class LocalPart(TokenList):
+
+    token_type = 'local-part'
+    as_ew_allowed = False
+
+    @property
+    def value(self):
+        if self[0].token_type == "quoted-string":
+            return self[0].quoted_value
+        else:
+            return self[0].value
+
+    @property
+    def local_part(self):
+        # Strip whitespace from front, back, and around dots.
+        res = [DOT]
+        last = DOT
+        last_is_tl = False
+        for tok in self[0] + [DOT]:
+            if tok.token_type == 'cfws':
+                continue
+            if (last_is_tl and tok.token_type == 'dot' and
+                    last[-1].token_type == 'cfws'):
+                res[-1] = TokenList(last[:-1])
+            is_tl = isinstance(tok, TokenList)
+            if (is_tl and last.token_type == 'dot' and
+                    tok[0].token_type == 'cfws'):
+                res.append(TokenList(tok[1:]))
+            else:
+                res.append(tok)
+            last = res[-1]
+            last_is_tl = is_tl
+        res = TokenList(res[1:-1])
+        return res.value
+
+
+class DomainLiteral(TokenList):
+
+    token_type = 'domain-literal'
+    as_ew_allowed = False
+
+    @property
+    def domain(self):
+        return ''.join(super().value.split())
+
+    @property
+    def ip(self):
+        for x in self:
+            if x.token_type == 'ptext':
+                return x.value
+
+
+class MIMEVersion(TokenList):
+
+    token_type = 'mime-version'
+    major = None
+    minor = None
+
+
+class Parameter(TokenList):
+
+    token_type = 'parameter'
+    sectioned = False
+    extended = False
+    charset = 'us-ascii'
+
+    @property
+    def section_number(self):
+        # Because the first token, the attribute (name) eats CFWS, the second
+        # token is always the section if there is one.
+        return self[1].number if self.sectioned else 0
+
+    @property
+    def param_value(self):
+        # This is part of the "handle quoted extended parameters" hack.
+        for token in self:
+            if token.token_type == 'value':
+                return token.stripped_value
+            if token.token_type == 'quoted-string':
+                for token in token:
+                    if token.token_type == 'bare-quoted-string':
+                        for token in token:
+                            if token.token_type == 'value':
+                                return token.stripped_value
+        return ''
+
+
+class InvalidParameter(Parameter):
+
+    token_type = 'invalid-parameter'
+
+
+class Attribute(TokenList):
+
+    token_type = 'attribute'
+
+    @property
+    def stripped_value(self):
+        for token in self:
+            if token.token_type.endswith('attrtext'):
+                return token.value
+
+class Section(TokenList):
+
+    token_type = 'section'
+    number = None
+
+
+class Value(TokenList):
+
+    token_type = 'value'
+
+    @property
+    def stripped_value(self):
+        token = self[0]
+        if token.token_type == 'cfws':
+            token = self[1]
+        if token.token_type.endswith(
+                ('quoted-string', 'attribute', 'extended-attribute')):
+            return token.stripped_value
+        return self.value
+
+
+class MimeParameters(TokenList):
+
+    token_type = 'mime-parameters'
+    syntactic_break = False
+
+    @property
+    def params(self):
+        # The RFC specifically states that the ordering of parameters is not
+        # guaranteed and may be reordered by the transport layer.  So we have
+        # to assume the RFC 2231 pieces can come in any order.  However, we
+        # output them in the order that we first see a given name, which gives
+        # us a stable __str__.
+        params = {}  # Using order preserving dict from Python 3.7+
+        for token in self:
+            if not token.token_type.endswith('parameter'):
+                continue
+            if token[0].token_type != 'attribute':
+                continue
+            name = token[0].value.strip()
+            if name not in params:
+                params[name] = []
+            params[name].append((token.section_number, token))
+        for name, parts in params.items():
+            parts = sorted(parts, key=itemgetter(0))
+            first_param = parts[0][1]
+            charset = first_param.charset
+            # Our arbitrary error recovery is to ignore duplicate parameters,
+            # to use appearance order if there are duplicate rfc 2231 parts,
+            # and to ignore gaps.  This mimics the error recovery of get_param.
+            if not first_param.extended and len(parts) > 1:
+                if parts[1][0] == 0:
+                    parts[1][1].defects.append(errors.InvalidHeaderDefect(
+                        'duplicate parameter name; duplicate(s) ignored'))
+                    parts = parts[:1]
+                # Else assume the *0* was missing...note that this is different
+                # from get_param, but we registered a defect for this earlier.
+            value_parts = []
+            i = 0
+            for section_number, param in parts:
+                if section_number != i:
+                    # We could get fancier here and look for a complete
+                    # duplicate extended parameter and ignore the second one
+                    # seen.  But we're not doing that.  The old code didn't.
+                    if not param.extended:
+                        param.defects.append(errors.InvalidHeaderDefect(
+                            'duplicate parameter name; duplicate ignored'))
+                        continue
+                    else:
+                        param.defects.append(errors.InvalidHeaderDefect(
+                            "inconsistent RFC2231 parameter numbering"))
+                i += 1
+                value = param.param_value
+                if param.extended:
+                    try:
+                        value = urllib.parse.unquote_to_bytes(value)
+                    except UnicodeEncodeError:
+                        # source had surrogate escaped bytes.  What we do now
+                        # is a bit of an open question.  I'm not sure this is
+                        # the best choice, but it is what the old algorithm did
+                        value = urllib.parse.unquote(value, encoding='latin-1')
+                    else:
+                        try:
+                            value = value.decode(charset, 'surrogateescape')
+                        except (LookupError, UnicodeEncodeError):
+                            # XXX: there should really be a custom defect for
+                            # unknown character set to make it easy to find,
+                            # because otherwise unknown charset is a silent
+                            # failure.
+                            value = value.decode('us-ascii', 'surrogateescape')
+                        if utils._has_surrogates(value):
+                            param.defects.append(errors.UndecodableBytesDefect())
+                value_parts.append(value)
+            value = ''.join(value_parts)
+            yield name, value
+
+    def __str__(self):
+        params = []
+        for name, value in self.params:
+            if value:
+                params.append('{}={}'.format(name, quote_string(value)))
+            else:
+                params.append(name)
+        params = '; '.join(params)
+        return ' ' + params if params else ''
+
+
+class ParameterizedHeaderValue(TokenList):
+
+    # Set this false so that the value doesn't wind up on a new line even
+    # if it and the parameters would fit there but not on the first line.
+    syntactic_break = False
+
+    @property
+    def params(self):
+        for token in reversed(self):
+            if token.token_type == 'mime-parameters':
+                return token.params
+        return {}
+
+
+class ContentType(ParameterizedHeaderValue):
+    token_type = 'content-type'
+    as_ew_allowed = False
+    maintype = 'text'
+    subtype = 'plain'
+
+
+class ContentDisposition(ParameterizedHeaderValue):
+    token_type = 'content-disposition'
+    as_ew_allowed = False
+    content_disposition = None
+
+
+class ContentTransferEncoding(TokenList):
+    token_type = 'content-transfer-encoding'
+    as_ew_allowed = False
+    cte = '7bit'
+
+
+class HeaderLabel(TokenList):
+    token_type = 'header-label'
+    as_ew_allowed = False
+
+
+class MsgID(TokenList):
+    token_type = 'msg-id'
+    as_ew_allowed = False
+
+    def fold(self, policy):
+        # message-id tokens may not be folded.
+        return str(self) + policy.linesep
+
+
+class MessageID(MsgID):
+    token_type = 'message-id'
+
+
+class InvalidMessageID(MessageID):
+    token_type = 'invalid-message-id'
+
+
+class Header(TokenList):
+    token_type = 'header'
+
+
+#
+# Terminal classes and instances
+#
+
+class Terminal(str):
+
+    as_ew_allowed = True
+    ew_combine_allowed = True
+    syntactic_break = True
+
+    def __new__(cls, value, token_type):
+        self = super().__new__(cls, value)
+        self.token_type = token_type
+        self.defects = []
+        return self
+
+    def __repr__(self):
+        return "{}({})".format(self.__class__.__name__, super().__repr__())
+
+    def pprint(self):
+        print(self.__class__.__name__ + '/' + self.token_type)
+
+    @property
+    def all_defects(self):
+        return list(self.defects)
+
+    def _pp(self, indent=''):
+        return ["{}{}/{}({}){}".format(
+            indent,
+            self.__class__.__name__,
+            self.token_type,
+            super().__repr__(),
+            '' if not self.defects else ' {}'.format(self.defects),
+            )]
+
+    def pop_trailing_ws(self):
+        # This terminates the recursion.
+        return None
+
+    @property
+    def comments(self):
+        return []
+
+    def __getnewargs__(self):
+        return(str(self), self.token_type)
+
+
+class WhiteSpaceTerminal(Terminal):
+
+    @property
+    def value(self):
+        return ' '
+
+    def startswith_fws(self):
+        return True
+
+
+class ValueTerminal(Terminal):
+
+    @property
+    def value(self):
+        return self
+
+    def startswith_fws(self):
+        return False
+
+
+class EWWhiteSpaceTerminal(WhiteSpaceTerminal):
+
+    @property
+    def value(self):
+        return ''
+
+    def __str__(self):
+        return ''
+
+
+class _InvalidEwError(errors.HeaderParseError):
+    """Invalid encoded word found while parsing headers."""
+
+
+# XXX these need to become classes and used as instances so
+# that a program can't change them in a parse tree and screw
+# up other parse trees.  Maybe should have  tests for that, too.
+DOT = ValueTerminal('.', 'dot')
+ListSeparator = ValueTerminal(',', 'list-separator')
+RouteComponentMarker = ValueTerminal('@', 'route-component-marker')
+
+#
+# Parser
+#
+
+# Parse strings according to RFC822/2047/2822/5322 rules.
+#
+# This is a stateless parser.  Each get_XXX function accepts a string and
+# returns either a Terminal or a TokenList representing the RFC object named
+# by the method and a string containing the remaining unparsed characters
+# from the input.  Thus a parser method consumes the next syntactic construct
+# of a given type and returns a token representing the construct plus the
+# unparsed remainder of the input string.
+#
+# For example, if the first element of a structured header is a 'phrase',
+# then:
+#
+#     phrase, value = get_phrase(value)
+#
+# returns the complete phrase from the start of the string value, plus any
+# characters left in the string after the phrase is removed.
+
+_wsp_splitter = re.compile(r'([{}]+)'.format(''.join(WSP))).split
+_non_atom_end_matcher = re.compile(r"[^{}]+".format(
+    re.escape(''.join(ATOM_ENDS)))).match
+_non_printable_finder = re.compile(r"[\x00-\x20\x7F]").findall
+_non_token_end_matcher = re.compile(r"[^{}]+".format(
+    re.escape(''.join(TOKEN_ENDS)))).match
+_non_attribute_end_matcher = re.compile(r"[^{}]+".format(
+    re.escape(''.join(ATTRIBUTE_ENDS)))).match
+_non_extended_attribute_end_matcher = re.compile(r"[^{}]+".format(
+    re.escape(''.join(EXTENDED_ATTRIBUTE_ENDS)))).match
+
+def _validate_xtext(xtext):
+    """If input token contains ASCII non-printables, register a defect."""
+
+    non_printables = _non_printable_finder(xtext)
+    if non_printables:
+        xtext.defects.append(errors.NonPrintableDefect(non_printables))
+    if utils._has_surrogates(xtext):
+        xtext.defects.append(errors.UndecodableBytesDefect(
+            "Non-ASCII characters found in header token"))
+
+def _get_ptext_to_endchars(value, endchars):
+    """Scan printables/quoted-pairs until endchars and return unquoted ptext.
+
+    This function turns a run of qcontent, ccontent-without-comments, or
+    dtext-with-quoted-printables into a single string by unquoting any
+    quoted printables.  It returns the string, the remaining value, and
+    a flag that is True iff there were any quoted printables decoded.
+
+    """
+    fragment, *remainder = _wsp_splitter(value, 1)
+    vchars = []
+    escape = False
+    had_qp = False
+    for pos in range(len(fragment)):
+        if fragment[pos] == '\\':
+            if escape:
+                escape = False
+                had_qp = True
+            else:
+                escape = True
+                continue
+        if escape:
+            escape = False
+        elif fragment[pos] in endchars:
+            break
+        vchars.append(fragment[pos])
+    else:
+        pos = pos + 1
+    return ''.join(vchars), ''.join([fragment[pos:]] + remainder), had_qp
+
+def get_fws(value):
+    """FWS = 1*WSP
+
+    This isn't the RFC definition.  We're using fws to represent tokens where
+    folding can be done, but when we are parsing the *un*folding has already
+    been done so we don't need to watch out for CRLF.
+
+    """
+    newvalue = value.lstrip()
+    fws = WhiteSpaceTerminal(value[:len(value)-len(newvalue)], 'fws')
+    return fws, newvalue
+
+def get_encoded_word(value):
+    """ encoded-word = "=?" charset "?" encoding "?" encoded-text "?="
+
+    """
+    ew = EncodedWord()
+    if not value.startswith('=?'):
+        raise errors.HeaderParseError(
+            "expected encoded word but found {}".format(value))
+    tok, *remainder = value[2:].split('?=', 1)
+    if tok == value[2:]:
+        raise errors.HeaderParseError(
+            "expected encoded word but found {}".format(value))
+    remstr = ''.join(remainder)
+    if (len(remstr) > 1 and
+        remstr[0] in hexdigits and
+        remstr[1] in hexdigits and
+        tok.count('?') < 2):
+        # The ? after the CTE was followed by an encoded word escape (=XX).
+        rest, *remainder = remstr.split('?=', 1)
+        tok = tok + '?=' + rest
+    if len(tok.split()) > 1:
+        ew.defects.append(errors.InvalidHeaderDefect(
+            "whitespace inside encoded word"))
+    ew.cte = value
+    value = ''.join(remainder)
+    try:
+        text, charset, lang, defects = _ew.decode('=?' + tok + '?=')
+    except (ValueError, KeyError):
+        raise _InvalidEwError(
+            "encoded word format invalid: '{}'".format(ew.cte))
+    ew.charset = charset
+    ew.lang = lang
+    ew.defects.extend(defects)
+    while text:
+        if text[0] in WSP:
+            token, text = get_fws(text)
+            ew.append(token)
+            continue
+        chars, *remainder = _wsp_splitter(text, 1)
+        vtext = ValueTerminal(chars, 'vtext')
+        _validate_xtext(vtext)
+        ew.append(vtext)
+        text = ''.join(remainder)
+    # Encoded words should be followed by a WS
+    if value and value[0] not in WSP:
+        ew.defects.append(errors.InvalidHeaderDefect(
+            "missing trailing whitespace after encoded-word"))
+    return ew, value
+
+def get_unstructured(value):
+    """unstructured = (*([FWS] vchar) *WSP) / obs-unstruct
+       obs-unstruct = *((*LF *CR *(obs-utext) *LF *CR)) / FWS)
+       obs-utext = %d0 / obs-NO-WS-CTL / LF / CR
+
+       obs-NO-WS-CTL is control characters except WSP/CR/LF.
+
+    So, basically, we have printable runs, plus control characters or nulls in
+    the obsolete syntax, separated by whitespace.  Since RFC 2047 uses the
+    obsolete syntax in its specification, but requires whitespace on either
+    side of the encoded words, I can see no reason to need to separate the
+    non-printable-non-whitespace from the printable runs if they occur, so we
+    parse this into xtext tokens separated by WSP tokens.
+
+    Because an 'unstructured' value must by definition constitute the entire
+    value, this 'get' routine does not return a remaining value, only the
+    parsed TokenList.
+
+    """
+    # XXX: but what about bare CR and LF?  They might signal the start or
+    # end of an encoded word.  YAGNI for now, since our current parsers
+    # will never send us strings with bare CR or LF.
+
+    unstructured = UnstructuredTokenList()
+    while value:
+        if value[0] in WSP:
+            token, value = get_fws(value)
+            unstructured.append(token)
+            continue
+        valid_ew = True
+        if value.startswith('=?'):
+            try:
+                token, value = get_encoded_word(value)
+            except _InvalidEwError:
+                valid_ew = False
+            except errors.HeaderParseError:
+                # XXX: Need to figure out how to register defects when
+                # appropriate here.
+                pass
+            else:
+                have_ws = True
+                if len(unstructured) > 0:
+                    if unstructured[-1].token_type != 'fws':
+                        unstructured.defects.append(errors.InvalidHeaderDefect(
+                            "missing whitespace before encoded word"))
+                        have_ws = False
+                if have_ws and len(unstructured) > 1:
+                    if unstructured[-2].token_type == 'encoded-word':
+                        unstructured[-1] = EWWhiteSpaceTerminal(
+                            unstructured[-1], 'fws')
+                unstructured.append(token)
+                continue
+        tok, *remainder = _wsp_splitter(value, 1)
+        # Split in the middle of an atom if there is a rfc2047 encoded word
+        # which does not have WSP on both sides. The defect will be registered
+        # the next time through the loop.
+        # This needs to only be performed when the encoded word is valid;
+        # otherwise, performing it on an invalid encoded word can cause
+        # the parser to go in an infinite loop.
+        if valid_ew and rfc2047_matcher.search(tok):
+            tok, *remainder = value.partition('=?')
+        vtext = ValueTerminal(tok, 'vtext')
+        _validate_xtext(vtext)
+        unstructured.append(vtext)
+        value = ''.join(remainder)
+    return unstructured
+
+def get_qp_ctext(value):
+    r"""ctext = <printable ascii except \ ( )>
+
+    This is not the RFC ctext, since we are handling nested comments in comment
+    and unquoting quoted-pairs here.  We allow anything except the '()'
+    characters, but if we find any ASCII other than the RFC defined printable
+    ASCII, a NonPrintableDefect is added to the token's defects list.  Since
+    quoted pairs are converted to their unquoted values, what is returned is
+    a 'ptext' token.  In this case it is a WhiteSpaceTerminal, so it's value
+    is ' '.
+
+    """
+    ptext, value, _ = _get_ptext_to_endchars(value, '()')
+    ptext = WhiteSpaceTerminal(ptext, 'ptext')
+    _validate_xtext(ptext)
+    return ptext, value
+
+def get_qcontent(value):
+    """qcontent = qtext / quoted-pair
+
+    We allow anything except the DQUOTE character, but if we find any ASCII
+    other than the RFC defined printable ASCII, a NonPrintableDefect is
+    added to the token's defects list.  Any quoted pairs are converted to their
+    unquoted values, so what is returned is a 'ptext' token.  In this case it
+    is a ValueTerminal.
+
+    """
+    ptext, value, _ = _get_ptext_to_endchars(value, '"')
+    ptext = ValueTerminal(ptext, 'ptext')
+    _validate_xtext(ptext)
+    return ptext, value
+
+def get_atext(value):
+    """atext = <matches _atext_matcher>
+
+    We allow any non-ATOM_ENDS in atext, but add an InvalidATextDefect to
+    the token's defects list if we find non-atext characters.
+    """
+    m = _non_atom_end_matcher(value)
+    if not m:
+        raise errors.HeaderParseError(
+            "expected atext but found '{}'".format(value))
+    atext = m.group()
+    value = value[len(atext):]
+    atext = ValueTerminal(atext, 'atext')
+    _validate_xtext(atext)
+    return atext, value
+
+def get_bare_quoted_string(value):
+    """bare-quoted-string = DQUOTE *([FWS] qcontent) [FWS] DQUOTE
+
+    A quoted-string without the leading or trailing white space.  Its
+    value is the text between the quote marks, with whitespace
+    preserved and quoted pairs decoded.
+    """
+    if value[0] != '"':
+        raise errors.HeaderParseError(
+            "expected '\"' but found '{}'".format(value))
+    bare_quoted_string = BareQuotedString()
+    value = value[1:]
+    if value and value[0] == '"':
+        token, value = get_qcontent(value)
+        bare_quoted_string.append(token)
+    while value and value[0] != '"':
+        if value[0] in WSP:
+            token, value = get_fws(value)
+        elif value[:2] == '=?':
+            valid_ew = False
+            try:
+                token, value = get_encoded_word(value)
+                bare_quoted_string.defects.append(errors.InvalidHeaderDefect(
+                    "encoded word inside quoted string"))
+                valid_ew = True
+            except errors.HeaderParseError:
+                token, value = get_qcontent(value)
+            # Collapse the whitespace between two encoded words that occur in a
+            # bare-quoted-string.
+            if valid_ew and len(bare_quoted_string) > 1:
+                if (bare_quoted_string[-1].token_type == 'fws' and
+                        bare_quoted_string[-2].token_type == 'encoded-word'):
+                    bare_quoted_string[-1] = EWWhiteSpaceTerminal(
+                        bare_quoted_string[-1], 'fws')
+        else:
+            token, value = get_qcontent(value)
+        bare_quoted_string.append(token)
+    if not value:
+        bare_quoted_string.defects.append(errors.InvalidHeaderDefect(
+            "end of header inside quoted string"))
+        return bare_quoted_string, value
+    return bare_quoted_string, value[1:]
+
+def get_comment(value):
+    """comment = "(" *([FWS] ccontent) [FWS] ")"
+       ccontent = ctext / quoted-pair / comment
+
+    We handle nested comments here, and quoted-pair in our qp-ctext routine.
+    """
+    if value and value[0] != '(':
+        raise errors.HeaderParseError(
+            "expected '(' but found '{}'".format(value))
+    comment = Comment()
+    value = value[1:]
+    while value and value[0] != ")":
+        if value[0] in WSP:
+            token, value = get_fws(value)
+        elif value[0] == '(':
+            token, value = get_comment(value)
+        else:
+            token, value = get_qp_ctext(value)
+        comment.append(token)
+    if not value:
+        comment.defects.append(errors.InvalidHeaderDefect(
+            "end of header inside comment"))
+        return comment, value
+    return comment, value[1:]
+
+def get_cfws(value):
+    """CFWS = (1*([FWS] comment) [FWS]) / FWS
+
+    """
+    cfws = CFWSList()
+    while value and value[0] in CFWS_LEADER:
+        if value[0] in WSP:
+            token, value = get_fws(value)
+        else:
+            token, value = get_comment(value)
+        cfws.append(token)
+    return cfws, value
+
+def get_quoted_string(value):
+    """quoted-string = [CFWS] <bare-quoted-string> [CFWS]
+
+    'bare-quoted-string' is an intermediate class defined by this
+    parser and not by the RFC grammar.  It is the quoted string
+    without any attached CFWS.
+    """
+    quoted_string = QuotedString()
+    if value and value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        quoted_string.append(token)
+    token, value = get_bare_quoted_string(value)
+    quoted_string.append(token)
+    if value and value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        quoted_string.append(token)
+    return quoted_string, value
+
+def get_atom(value):
+    """atom = [CFWS] 1*atext [CFWS]
+
+    An atom could be an rfc2047 encoded word.
+    """
+    atom = Atom()
+    if value and value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        atom.append(token)
+    if value and value[0] in ATOM_ENDS:
+        raise errors.HeaderParseError(
+            "expected atom but found '{}'".format(value))
+    if value.startswith('=?'):
+        try:
+            token, value = get_encoded_word(value)
+        except errors.HeaderParseError:
+            # XXX: need to figure out how to register defects when
+            # appropriate here.
+            token, value = get_atext(value)
+    else:
+        token, value = get_atext(value)
+    atom.append(token)
+    if value and value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        atom.append(token)
+    return atom, value
+
+def get_dot_atom_text(value):
+    """ dot-text = 1*atext *("." 1*atext)
+
+    """
+    dot_atom_text = DotAtomText()
+    if not value or value[0] in ATOM_ENDS:
+        raise errors.HeaderParseError("expected atom at a start of "
+            "dot-atom-text but found '{}'".format(value))
+    while value and value[0] not in ATOM_ENDS:
+        token, value = get_atext(value)
+        dot_atom_text.append(token)
+        if value and value[0] == '.':
+            dot_atom_text.append(DOT)
+            value = value[1:]
+    if dot_atom_text[-1] is DOT:
+        raise errors.HeaderParseError("expected atom at end of dot-atom-text "
+            "but found '{}'".format('.'+value))
+    return dot_atom_text, value
+
+def get_dot_atom(value):
+    """ dot-atom = [CFWS] dot-atom-text [CFWS]
+
+    Any place we can have a dot atom, we could instead have an rfc2047 encoded
+    word.
+    """
+    dot_atom = DotAtom()
+    if value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        dot_atom.append(token)
+    if value.startswith('=?'):
+        try:
+            token, value = get_encoded_word(value)
+        except errors.HeaderParseError:
+            # XXX: need to figure out how to register defects when
+            # appropriate here.
+            token, value = get_dot_atom_text(value)
+    else:
+        token, value = get_dot_atom_text(value)
+    dot_atom.append(token)
+    if value and value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        dot_atom.append(token)
+    return dot_atom, value
+
+def get_word(value):
+    """word = atom / quoted-string
+
+    Either atom or quoted-string may start with CFWS.  We have to peel off this
+    CFWS first to determine which type of word to parse.  Afterward we splice
+    the leading CFWS, if any, into the parsed sub-token.
+
+    If neither an atom or a quoted-string is found before the next special, a
+    HeaderParseError is raised.
+
+    The token returned is either an Atom or a QuotedString, as appropriate.
+    This means the 'word' level of the formal grammar is not represented in the
+    parse tree; this is because having that extra layer when manipulating the
+    parse tree is more confusing than it is helpful.
+
+    """
+    if value[0] in CFWS_LEADER:
+        leader, value = get_cfws(value)
+    else:
+        leader = None
+    if not value:
+        raise errors.HeaderParseError(
+            "Expected 'atom' or 'quoted-string' but found nothing.")
+    if value[0]=='"':
+        token, value = get_quoted_string(value)
+    elif value[0] in SPECIALS:
+        raise errors.HeaderParseError("Expected 'atom' or 'quoted-string' "
+                                      "but found '{}'".format(value))
+    else:
+        token, value = get_atom(value)
+    if leader is not None:
+        token[:0] = [leader]
+    return token, value
+
+def get_phrase(value):
+    """ phrase = 1*word / obs-phrase
+        obs-phrase = word *(word / "." / CFWS)
+
+    This means a phrase can be a sequence of words, periods, and CFWS in any
+    order as long as it starts with at least one word.  If anything other than
+    words is detected, an ObsoleteHeaderDefect is added to the token's defect
+    list.  We also accept a phrase that starts with CFWS followed by a dot;
+    this is registered as an InvalidHeaderDefect, since it is not supported by
+    even the obsolete grammar.
+
+    """
+    phrase = Phrase()
+    try:
+        token, value = get_word(value)
+        phrase.append(token)
+    except errors.HeaderParseError:
+        phrase.defects.append(errors.InvalidHeaderDefect(
+            "phrase does not start with word"))
+    while value and value[0] not in PHRASE_ENDS:
+        if value[0]=='.':
+            phrase.append(DOT)
+            phrase.defects.append(errors.ObsoleteHeaderDefect(
+                "period in 'phrase'"))
+            value = value[1:]
+        else:
+            try:
+                token, value = get_word(value)
+            except errors.HeaderParseError:
+                if value[0] in CFWS_LEADER:
+                    token, value = get_cfws(value)
+                    phrase.defects.append(errors.ObsoleteHeaderDefect(
+                        "comment found without atom"))
+                else:
+                    raise
+            phrase.append(token)
+    return phrase, value
+
+def get_local_part(value):
+    """ local-part = dot-atom / quoted-string / obs-local-part
+
+    """
+    local_part = LocalPart()
+    leader = None
+    if value[0] in CFWS_LEADER:
+        leader, value = get_cfws(value)
+    if not value:
+        raise errors.HeaderParseError(
+            "expected local-part but found '{}'".format(value))
+    try:
+        token, value = get_dot_atom(value)
+    except errors.HeaderParseError:
+        try:
+            token, value = get_word(value)
+        except errors.HeaderParseError:
+            if value[0] != '\\' and value[0] in PHRASE_ENDS:
+                raise
+            token = TokenList()
+    if leader is not None:
+        token[:0] = [leader]
+    local_part.append(token)
+    if value and (value[0]=='\\' or value[0] not in PHRASE_ENDS):
+        obs_local_part, value = get_obs_local_part(str(local_part) + value)
+        if obs_local_part.token_type == 'invalid-obs-local-part':
+            local_part.defects.append(errors.InvalidHeaderDefect(
+                "local-part is not dot-atom, quoted-string, or obs-local-part"))
+        else:
+            local_part.defects.append(errors.ObsoleteHeaderDefect(
+                "local-part is not a dot-atom (contains CFWS)"))
+        local_part[0] = obs_local_part
+    try:
+        local_part.value.encode('ascii')
+    except UnicodeEncodeError:
+        local_part.defects.append(errors.NonASCIILocalPartDefect(
+                "local-part contains non-ASCII characters)"))
+    return local_part, value
+
+def get_obs_local_part(value):
+    """ obs-local-part = word *("." word)
+    """
+    obs_local_part = ObsLocalPart()
+    last_non_ws_was_dot = False
+    while value and (value[0]=='\\' or value[0] not in PHRASE_ENDS):
+        if value[0] == '.':
+            if last_non_ws_was_dot:
+                obs_local_part.defects.append(errors.InvalidHeaderDefect(
+                    "invalid repeated '.'"))
+            obs_local_part.append(DOT)
+            last_non_ws_was_dot = True
+            value = value[1:]
+            continue
+        elif value[0]=='\\':
+            obs_local_part.append(ValueTerminal(value[0],
+                                                'misplaced-special'))
+            value = value[1:]
+            obs_local_part.defects.append(errors.InvalidHeaderDefect(
+                "'\\' character outside of quoted-string/ccontent"))
+            last_non_ws_was_dot = False
+            continue
+        if obs_local_part and obs_local_part[-1].token_type != 'dot':
+            obs_local_part.defects.append(errors.InvalidHeaderDefect(
+                "missing '.' between words"))
+        try:
+            token, value = get_word(value)
+            last_non_ws_was_dot = False
+        except errors.HeaderParseError:
+            if value[0] not in CFWS_LEADER:
+                raise
+            token, value = get_cfws(value)
+        obs_local_part.append(token)
+    if (obs_local_part[0].token_type == 'dot' or
+            obs_local_part[0].token_type=='cfws' and
+            obs_local_part[1].token_type=='dot'):
+        obs_local_part.defects.append(errors.InvalidHeaderDefect(
+            "Invalid leading '.' in local part"))
+    if (obs_local_part[-1].token_type == 'dot' or
+            obs_local_part[-1].token_type=='cfws' and
+            obs_local_part[-2].token_type=='dot'):
+        obs_local_part.defects.append(errors.InvalidHeaderDefect(
+            "Invalid trailing '.' in local part"))
+    if obs_local_part.defects:
+        obs_local_part.token_type = 'invalid-obs-local-part'
+    return obs_local_part, value
+
+def get_dtext(value):
+    r""" dtext = <printable ascii except \ [ ]> / obs-dtext
+        obs-dtext = obs-NO-WS-CTL / quoted-pair
+
+    We allow anything except the excluded characters, but if we find any
+    ASCII other than the RFC defined printable ASCII, a NonPrintableDefect is
+    added to the token's defects list.  Quoted pairs are converted to their
+    unquoted values, so what is returned is a ptext token, in this case a
+    ValueTerminal.  If there were quoted-printables, an ObsoleteHeaderDefect is
+    added to the returned token's defect list.
+
+    """
+    ptext, value, had_qp = _get_ptext_to_endchars(value, '[]')
+    ptext = ValueTerminal(ptext, 'ptext')
+    if had_qp:
+        ptext.defects.append(errors.ObsoleteHeaderDefect(
+            "quoted printable found in domain-literal"))
+    _validate_xtext(ptext)
+    return ptext, value
+
+def _check_for_early_dl_end(value, domain_literal):
+    if value:
+        return False
+    domain_literal.append(errors.InvalidHeaderDefect(
+        "end of input inside domain-literal"))
+    domain_literal.append(ValueTerminal(']', 'domain-literal-end'))
+    return True
+
+def get_domain_literal(value):
+    """ domain-literal = [CFWS] "[" *([FWS] dtext) [FWS] "]" [CFWS]
+
+    """
+    domain_literal = DomainLiteral()
+    if value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        domain_literal.append(token)
+    if not value:
+        raise errors.HeaderParseError("expected domain-literal")
+    if value[0] != '[':
+        raise errors.HeaderParseError("expected '[' at start of domain-literal "
+                "but found '{}'".format(value))
+    value = value[1:]
+    if _check_for_early_dl_end(value, domain_literal):
+        return domain_literal, value
+    domain_literal.append(ValueTerminal('[', 'domain-literal-start'))
+    if value[0] in WSP:
+        token, value = get_fws(value)
+        domain_literal.append(token)
+    token, value = get_dtext(value)
+    domain_literal.append(token)
+    if _check_for_early_dl_end(value, domain_literal):
+        return domain_literal, value
+    if value[0] in WSP:
+        token, value = get_fws(value)
+        domain_literal.append(token)
+    if _check_for_early_dl_end(value, domain_literal):
+        return domain_literal, value
+    if value[0] != ']':
+        raise errors.HeaderParseError("expected ']' at end of domain-literal "
+                "but found '{}'".format(value))
+    domain_literal.append(ValueTerminal(']', 'domain-literal-end'))
+    value = value[1:]
+    if value and value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        domain_literal.append(token)
+    return domain_literal, value
+
+def get_domain(value):
+    """ domain = dot-atom / domain-literal / obs-domain
+        obs-domain = atom *("." atom))
+
+    """
+    domain = Domain()
+    leader = None
+    if value[0] in CFWS_LEADER:
+        leader, value = get_cfws(value)
+    if not value:
+        raise errors.HeaderParseError(
+            "expected domain but found '{}'".format(value))
+    if value[0] == '[':
+        token, value = get_domain_literal(value)
+        if leader is not None:
+            token[:0] = [leader]
+        domain.append(token)
+        return domain, value
+    try:
+        token, value = get_dot_atom(value)
+    except errors.HeaderParseError:
+        token, value = get_atom(value)
+    if value and value[0] == '@':
+        raise errors.HeaderParseError('Invalid Domain')
+    if leader is not None:
+        token[:0] = [leader]
+    domain.append(token)
+    if value and value[0] == '.':
+        domain.defects.append(errors.ObsoleteHeaderDefect(
+            "domain is not a dot-atom (contains CFWS)"))
+        if domain[0].token_type == 'dot-atom':
+            domain[:] = domain[0]
+        while value and value[0] == '.':
+            domain.append(DOT)
+            token, value = get_atom(value[1:])
+            domain.append(token)
+    return domain, value
+
+def get_addr_spec(value):
+    """ addr-spec = local-part "@" domain
+
+    """
+    addr_spec = AddrSpec()
+    token, value = get_local_part(value)
+    addr_spec.append(token)
+    if not value or value[0] != '@':
+        addr_spec.defects.append(errors.InvalidHeaderDefect(
+            "addr-spec local part with no domain"))
+        return addr_spec, value
+    addr_spec.append(ValueTerminal('@', 'address-at-symbol'))
+    token, value = get_domain(value[1:])
+    addr_spec.append(token)
+    return addr_spec, value
+
+def get_obs_route(value):
+    """ obs-route = obs-domain-list ":"
+        obs-domain-list = *(CFWS / ",") "@" domain *("," [CFWS] ["@" domain])
+
+        Returns an obs-route token with the appropriate sub-tokens (that is,
+        there is no obs-domain-list in the parse tree).
+    """
+    obs_route = ObsRoute()
+    while value and (value[0]==',' or value[0] in CFWS_LEADER):
+        if value[0] in CFWS_LEADER:
+            token, value = get_cfws(value)
+            obs_route.append(token)
+        elif value[0] == ',':
+            obs_route.append(ListSeparator)
+            value = value[1:]
+    if not value or value[0] != '@':
+        raise errors.HeaderParseError(
+            "expected obs-route domain but found '{}'".format(value))
+    obs_route.append(RouteComponentMarker)
+    token, value = get_domain(value[1:])
+    obs_route.append(token)
+    while value and value[0]==',':
+        obs_route.append(ListSeparator)
+        value = value[1:]
+        if not value:
+            break
+        if value[0] in CFWS_LEADER:
+            token, value = get_cfws(value)
+            obs_route.append(token)
+        if value[0] == '@':
+            obs_route.append(RouteComponentMarker)
+            token, value = get_domain(value[1:])
+            obs_route.append(token)
+    if not value:
+        raise errors.HeaderParseError("end of header while parsing obs-route")
+    if value[0] != ':':
+        raise errors.HeaderParseError( "expected ':' marking end of "
+            "obs-route but found '{}'".format(value))
+    obs_route.append(ValueTerminal(':', 'end-of-obs-route-marker'))
+    return obs_route, value[1:]
+
+def get_angle_addr(value):
+    """ angle-addr = [CFWS] "<" addr-spec ">" [CFWS] / obs-angle-addr
+        obs-angle-addr = [CFWS] "<" obs-route addr-spec ">" [CFWS]
+
+    """
+    angle_addr = AngleAddr()
+    if value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        angle_addr.append(token)
+    if not value or value[0] != '<':
+        raise errors.HeaderParseError(
+            "expected angle-addr but found '{}'".format(value))
+    angle_addr.append(ValueTerminal('<', 'angle-addr-start'))
+    value = value[1:]
+    # Although it is not legal per RFC5322, SMTP uses '<>' in certain
+    # circumstances.
+    if value[0] == '>':
+        angle_addr.append(ValueTerminal('>', 'angle-addr-end'))
+        angle_addr.defects.append(errors.InvalidHeaderDefect(
+            "null addr-spec in angle-addr"))
+        value = value[1:]
+        return angle_addr, value
+    try:
+        token, value = get_addr_spec(value)
+    except errors.HeaderParseError:
+        try:
+            token, value = get_obs_route(value)
+            angle_addr.defects.append(errors.ObsoleteHeaderDefect(
+                "obsolete route specification in angle-addr"))
+        except errors.HeaderParseError:
+            raise errors.HeaderParseError(
+                "expected addr-spec or obs-route but found '{}'".format(value))
+        angle_addr.append(token)
+        token, value = get_addr_spec(value)
+    angle_addr.append(token)
+    if value and value[0] == '>':
+        value = value[1:]
+    else:
+        angle_addr.defects.append(errors.InvalidHeaderDefect(
+            "missing trailing '>' on angle-addr"))
+    angle_addr.append(ValueTerminal('>', 'angle-addr-end'))
+    if value and value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        angle_addr.append(token)
+    return angle_addr, value
+
+def get_display_name(value):
+    """ display-name = phrase
+
+    Because this is simply a name-rule, we don't return a display-name
+    token containing a phrase, but rather a display-name token with
+    the content of the phrase.
+
+    """
+    display_name = DisplayName()
+    token, value = get_phrase(value)
+    display_name.extend(token[:])
+    display_name.defects = token.defects[:]
+    return display_name, value
+
+
+def get_name_addr(value):
+    """ name-addr = [display-name] angle-addr
+
+    """
+    name_addr = NameAddr()
+    # Both the optional display name and the angle-addr can start with cfws.
+    leader = None
+    if value[0] in CFWS_LEADER:
+        leader, value = get_cfws(value)
+        if not value:
+            raise errors.HeaderParseError(
+                "expected name-addr but found '{}'".format(leader))
+    if value[0] != '<':
+        if value[0] in PHRASE_ENDS:
+            raise errors.HeaderParseError(
+                "expected name-addr but found '{}'".format(value))
+        token, value = get_display_name(value)
+        if not value:
+            raise errors.HeaderParseError(
+                "expected name-addr but found '{}'".format(token))
+        if leader is not None:
+            token[0][:0] = [leader]
+            leader = None
+        name_addr.append(token)
+    token, value = get_angle_addr(value)
+    if leader is not None:
+        token[:0] = [leader]
+    name_addr.append(token)
+    return name_addr, value
+
+def get_mailbox(value):
+    """ mailbox = name-addr / addr-spec
+
+    """
+    # The only way to figure out if we are dealing with a name-addr or an
+    # addr-spec is to try parsing each one.
+    mailbox = Mailbox()
+    try:
+        token, value = get_name_addr(value)
+    except errors.HeaderParseError:
+        try:
+            token, value = get_addr_spec(value)
+        except errors.HeaderParseError:
+            raise errors.HeaderParseError(
+                "expected mailbox but found '{}'".format(value))
+    if any(isinstance(x, errors.InvalidHeaderDefect)
+                       for x in token.all_defects):
+        mailbox.token_type = 'invalid-mailbox'
+    mailbox.append(token)
+    return mailbox, value
+
+def get_invalid_mailbox(value, endchars):
+    """ Read everything up to one of the chars in endchars.
+
+    This is outside the formal grammar.  The InvalidMailbox TokenList that is
+    returned acts like a Mailbox, but the data attributes are None.
+
+    """
+    invalid_mailbox = InvalidMailbox()
+    while value and value[0] not in endchars:
+        if value[0] in PHRASE_ENDS:
+            invalid_mailbox.append(ValueTerminal(value[0],
+                                                 'misplaced-special'))
+            value = value[1:]
+        else:
+            token, value = get_phrase(value)
+            invalid_mailbox.append(token)
+    return invalid_mailbox, value
+
+def get_mailbox_list(value):
+    """ mailbox-list = (mailbox *("," mailbox)) / obs-mbox-list
+        obs-mbox-list = *([CFWS] ",") mailbox *("," [mailbox / CFWS])
+
+    For this routine we go outside the formal grammar in order to improve error
+    handling.  We recognize the end of the mailbox list only at the end of the
+    value or at a ';' (the group terminator).  This is so that we can turn
+    invalid mailboxes into InvalidMailbox tokens and continue parsing any
+    remaining valid mailboxes.  We also allow all mailbox entries to be null,
+    and this condition is handled appropriately at a higher level.
+
+    """
+    mailbox_list = MailboxList()
+    while value and value[0] != ';':
+        try:
+            token, value = get_mailbox(value)
+            mailbox_list.append(token)
+        except errors.HeaderParseError:
+            leader = None
+            if value[0] in CFWS_LEADER:
+                leader, value = get_cfws(value)
+                if not value or value[0] in ',;':
+                    mailbox_list.append(leader)
+                    mailbox_list.defects.append(errors.ObsoleteHeaderDefect(
+                        "empty element in mailbox-list"))
+                else:
+                    token, value = get_invalid_mailbox(value, ',;')
+                    if leader is not None:
+                        token[:0] = [leader]
+                    mailbox_list.append(token)
+                    mailbox_list.defects.append(errors.InvalidHeaderDefect(
+                        "invalid mailbox in mailbox-list"))
+            elif value[0] == ',':
+                mailbox_list.defects.append(errors.ObsoleteHeaderDefect(
+                    "empty element in mailbox-list"))
+            else:
+                token, value = get_invalid_mailbox(value, ',;')
+                if leader is not None:
+                    token[:0] = [leader]
+                mailbox_list.append(token)
+                mailbox_list.defects.append(errors.InvalidHeaderDefect(
+                    "invalid mailbox in mailbox-list"))
+        if value and value[0] not in ',;':
+            # Crap after mailbox; treat it as an invalid mailbox.
+            # The mailbox info will still be available.
+            mailbox = mailbox_list[-1]
+            mailbox.token_type = 'invalid-mailbox'
+            token, value = get_invalid_mailbox(value, ',;')
+            mailbox.extend(token)
+            mailbox_list.defects.append(errors.InvalidHeaderDefect(
+                "invalid mailbox in mailbox-list"))
+        if value and value[0] == ',':
+            mailbox_list.append(ListSeparator)
+            value = value[1:]
+    return mailbox_list, value
+
+
+def get_group_list(value):
+    """ group-list = mailbox-list / CFWS / obs-group-list
+        obs-group-list = 1*([CFWS] ",") [CFWS]
+
+    """
+    group_list = GroupList()
+    if not value:
+        group_list.defects.append(errors.InvalidHeaderDefect(
+            "end of header before group-list"))
+        return group_list, value
+    leader = None
+    if value and value[0] in CFWS_LEADER:
+        leader, value = get_cfws(value)
+        if not value:
+            # This should never happen in email parsing, since CFWS-only is a
+            # legal alternative to group-list in a group, which is the only
+            # place group-list appears.
+            group_list.defects.append(errors.InvalidHeaderDefect(
+                "end of header in group-list"))
+            group_list.append(leader)
+            return group_list, value
+        if value[0] == ';':
+            group_list.append(leader)
+            return group_list, value
+    token, value = get_mailbox_list(value)
+    if len(token.all_mailboxes)==0:
+        if leader is not None:
+            group_list.append(leader)
+        group_list.extend(token)
+        group_list.defects.append(errors.ObsoleteHeaderDefect(
+            "group-list with empty entries"))
+        return group_list, value
+    if leader is not None:
+        token[:0] = [leader]
+    group_list.append(token)
+    return group_list, value
+
+def get_group(value):
+    """ group = display-name ":" [group-list] ";" [CFWS]
+
+    """
+    group = Group()
+    token, value = get_display_name(value)
+    if not value or value[0] != ':':
+        raise errors.HeaderParseError("expected ':' at end of group "
+            "display name but found '{}'".format(value))
+    group.append(token)
+    group.append(ValueTerminal(':', 'group-display-name-terminator'))
+    value = value[1:]
+    if value and value[0] == ';':
+        group.append(ValueTerminal(';', 'group-terminator'))
+        return group, value[1:]
+    token, value = get_group_list(value)
+    group.append(token)
+    if not value:
+        group.defects.append(errors.InvalidHeaderDefect(
+            "end of header in group"))
+    elif value[0] != ';':
+        raise errors.HeaderParseError(
+            "expected ';' at end of group but found {}".format(value))
+    group.append(ValueTerminal(';', 'group-terminator'))
+    value = value[1:]
+    if value and value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        group.append(token)
+    return group, value
+
+def get_address(value):
+    """ address = mailbox / group
+
+    Note that counter-intuitively, an address can be either a single address or
+    a list of addresses (a group).  This is why the returned Address object has
+    a 'mailboxes' attribute which treats a single address as a list of length
+    one.  When you need to differentiate between to two cases, extract the single
+    element, which is either a mailbox or a group token.
+
+    """
+    # The formal grammar isn't very helpful when parsing an address.  mailbox
+    # and group, especially when allowing for obsolete forms, start off very
+    # similarly.  It is only when you reach one of @, <, or : that you know
+    # what you've got.  So, we try each one in turn, starting with the more
+    # likely of the two.  We could perhaps make this more efficient by looking
+    # for a phrase and then branching based on the next character, but that
+    # would be a premature optimization.
+    address = Address()
+    try:
+        token, value = get_group(value)
+    except errors.HeaderParseError:
+        try:
+            token, value = get_mailbox(value)
+        except errors.HeaderParseError:
+            raise errors.HeaderParseError(
+                "expected address but found '{}'".format(value))
+    address.append(token)
+    return address, value
+
+def get_address_list(value):
+    """ address_list = (address *("," address)) / obs-addr-list
+        obs-addr-list = *([CFWS] ",") address *("," [address / CFWS])
+
+    We depart from the formal grammar here by continuing to parse until the end
+    of the input, assuming the input to be entirely composed of an
+    address-list.  This is always true in email parsing, and allows us
+    to skip invalid addresses to parse additional valid ones.
+
+    """
+    address_list = AddressList()
+    while value:
+        try:
+            token, value = get_address(value)
+            address_list.append(token)
+        except errors.HeaderParseError:
+            leader = None
+            if value[0] in CFWS_LEADER:
+                leader, value = get_cfws(value)
+                if not value or value[0] == ',':
+                    address_list.append(leader)
+                    address_list.defects.append(errors.ObsoleteHeaderDefect(
+                        "address-list entry with no content"))
+                else:
+                    token, value = get_invalid_mailbox(value, ',')
+                    if leader is not None:
+                        token[:0] = [leader]
+                    address_list.append(Address([token]))
+                    address_list.defects.append(errors.InvalidHeaderDefect(
+                        "invalid address in address-list"))
+            elif value[0] == ',':
+                address_list.defects.append(errors.ObsoleteHeaderDefect(
+                    "empty element in address-list"))
+            else:
+                token, value = get_invalid_mailbox(value, ',')
+                if leader is not None:
+                    token[:0] = [leader]
+                address_list.append(Address([token]))
+                address_list.defects.append(errors.InvalidHeaderDefect(
+                    "invalid address in address-list"))
+        if value and value[0] != ',':
+            # Crap after address; treat it as an invalid mailbox.
+            # The mailbox info will still be available.
+            mailbox = address_list[-1][0]
+            mailbox.token_type = 'invalid-mailbox'
+            token, value = get_invalid_mailbox(value, ',')
+            mailbox.extend(token)
+            address_list.defects.append(errors.InvalidHeaderDefect(
+                "invalid address in address-list"))
+        if value:  # Must be a , at this point.
+            address_list.append(ValueTerminal(',', 'list-separator'))
+            value = value[1:]
+    return address_list, value
+
+
+def get_no_fold_literal(value):
+    """ no-fold-literal = "[" *dtext "]"
+    """
+    no_fold_literal = NoFoldLiteral()
+    if not value:
+        raise errors.HeaderParseError(
+            "expected no-fold-literal but found '{}'".format(value))
+    if value[0] != '[':
+        raise errors.HeaderParseError(
+            "expected '[' at the start of no-fold-literal "
+            "but found '{}'".format(value))
+    no_fold_literal.append(ValueTerminal('[', 'no-fold-literal-start'))
+    value = value[1:]
+    token, value = get_dtext(value)
+    no_fold_literal.append(token)
+    if not value or value[0] != ']':
+        raise errors.HeaderParseError(
+            "expected ']' at the end of no-fold-literal "
+            "but found '{}'".format(value))
+    no_fold_literal.append(ValueTerminal(']', 'no-fold-literal-end'))
+    return no_fold_literal, value[1:]
+
+def get_msg_id(value):
+    """msg-id = [CFWS] "<" id-left '@' id-right  ">" [CFWS]
+       id-left = dot-atom-text / obs-id-left
+       id-right = dot-atom-text / no-fold-literal / obs-id-right
+       no-fold-literal = "[" *dtext "]"
+    """
+    msg_id = MsgID()
+    if value and value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        msg_id.append(token)
+    if not value or value[0] != '<':
+        raise errors.HeaderParseError(
+            "expected msg-id but found '{}'".format(value))
+    msg_id.append(ValueTerminal('<', 'msg-id-start'))
+    value = value[1:]
+    # Parse id-left.
+    try:
+        token, value = get_dot_atom_text(value)
+    except errors.HeaderParseError:
+        try:
+            # obs-id-left is same as local-part of add-spec.
+            token, value = get_obs_local_part(value)
+            msg_id.defects.append(errors.ObsoleteHeaderDefect(
+                "obsolete id-left in msg-id"))
+        except errors.HeaderParseError:
+            raise errors.HeaderParseError(
+                "expected dot-atom-text or obs-id-left"
+                " but found '{}'".format(value))
+    msg_id.append(token)
+    if not value or value[0] != '@':
+        msg_id.defects.append(errors.InvalidHeaderDefect(
+            "msg-id with no id-right"))
+        # Even though there is no id-right, if the local part
+        # ends with `>` let's just parse it too and return
+        # along with the defect.
+        if value and value[0] == '>':
+            msg_id.append(ValueTerminal('>', 'msg-id-end'))
+            value = value[1:]
+        return msg_id, value
+    msg_id.append(ValueTerminal('@', 'address-at-symbol'))
+    value = value[1:]
+    # Parse id-right.
+    try:
+        token, value = get_dot_atom_text(value)
+    except errors.HeaderParseError:
+        try:
+            token, value = get_no_fold_literal(value)
+        except errors.HeaderParseError:
+            try:
+                token, value = get_domain(value)
+                msg_id.defects.append(errors.ObsoleteHeaderDefect(
+                    "obsolete id-right in msg-id"))
+            except errors.HeaderParseError:
+                raise errors.HeaderParseError(
+                    "expected dot-atom-text, no-fold-literal or obs-id-right"
+                    " but found '{}'".format(value))
+    msg_id.append(token)
+    if value and value[0] == '>':
+        value = value[1:]
+    else:
+        msg_id.defects.append(errors.InvalidHeaderDefect(
+            "missing trailing '>' on msg-id"))
+    msg_id.append(ValueTerminal('>', 'msg-id-end'))
+    if value and value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        msg_id.append(token)
+    return msg_id, value
+
+
+def parse_message_id(value):
+    """message-id      =   "Message-ID:" msg-id CRLF
+    """
+    message_id = MessageID()
+    try:
+        token, value = get_msg_id(value)
+        message_id.append(token)
+    except errors.HeaderParseError as ex:
+        token = get_unstructured(value)
+        message_id = InvalidMessageID(token)
+        message_id.defects.append(
+            errors.InvalidHeaderDefect("Invalid msg-id: {!r}".format(ex)))
+    else:
+        # Value after parsing a valid msg_id should be None.
+        if value:
+            message_id.defects.append(errors.InvalidHeaderDefect(
+                "Unexpected {!r}".format(value)))
+
+    return message_id
+
+#
+# XXX: As I begin to add additional header parsers, I'm realizing we probably
+# have two level of parser routines: the get_XXX methods that get a token in
+# the grammar, and parse_XXX methods that parse an entire field value.  So
+# get_address_list above should really be a parse_ method, as probably should
+# be get_unstructured.
+#
+
+def parse_mime_version(value):
+    """ mime-version = [CFWS] 1*digit [CFWS] "." [CFWS] 1*digit [CFWS]
+
+    """
+    # The [CFWS] is implicit in the RFC 2045 BNF.
+    # XXX: This routine is a bit verbose, should factor out a get_int method.
+    mime_version = MIMEVersion()
+    if not value:
+        mime_version.defects.append(errors.HeaderMissingRequiredValue(
+            "Missing MIME version number (eg: 1.0)"))
+        return mime_version
+    if value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        mime_version.append(token)
+        if not value:
+            mime_version.defects.append(errors.HeaderMissingRequiredValue(
+                "Expected MIME version number but found only CFWS"))
+    digits = ''
+    while value and value[0] != '.' and value[0] not in CFWS_LEADER:
+        digits += value[0]
+        value = value[1:]
+    if not digits.isdigit():
+        mime_version.defects.append(errors.InvalidHeaderDefect(
+            "Expected MIME major version number but found {!r}".format(digits)))
+        mime_version.append(ValueTerminal(digits, 'xtext'))
+    else:
+        mime_version.major = int(digits)
+        mime_version.append(ValueTerminal(digits, 'digits'))
+    if value and value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        mime_version.append(token)
+    if not value or value[0] != '.':
+        if mime_version.major is not None:
+            mime_version.defects.append(errors.InvalidHeaderDefect(
+                "Incomplete MIME version; found only major number"))
+        if value:
+            mime_version.append(ValueTerminal(value, 'xtext'))
+        return mime_version
+    mime_version.append(ValueTerminal('.', 'version-separator'))
+    value = value[1:]
+    if value and value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        mime_version.append(token)
+    if not value:
+        if mime_version.major is not None:
+            mime_version.defects.append(errors.InvalidHeaderDefect(
+                "Incomplete MIME version; found only major number"))
+        return mime_version
+    digits = ''
+    while value and value[0] not in CFWS_LEADER:
+        digits += value[0]
+        value = value[1:]
+    if not digits.isdigit():
+        mime_version.defects.append(errors.InvalidHeaderDefect(
+            "Expected MIME minor version number but found {!r}".format(digits)))
+        mime_version.append(ValueTerminal(digits, 'xtext'))
+    else:
+        mime_version.minor = int(digits)
+        mime_version.append(ValueTerminal(digits, 'digits'))
+    if value and value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        mime_version.append(token)
+    if value:
+        mime_version.defects.append(errors.InvalidHeaderDefect(
+            "Excess non-CFWS text after MIME version"))
+        mime_version.append(ValueTerminal(value, 'xtext'))
+    return mime_version
+
+def get_invalid_parameter(value):
+    """ Read everything up to the next ';'.
+
+    This is outside the formal grammar.  The InvalidParameter TokenList that is
+    returned acts like a Parameter, but the data attributes are None.
+
+    """
+    invalid_parameter = InvalidParameter()
+    while value and value[0] != ';':
+        if value[0] in PHRASE_ENDS:
+            invalid_parameter.append(ValueTerminal(value[0],
+                                                   'misplaced-special'))
+            value = value[1:]
+        else:
+            token, value = get_phrase(value)
+            invalid_parameter.append(token)
+    return invalid_parameter, value
+
+def get_ttext(value):
+    """ttext = <matches _ttext_matcher>
+
+    We allow any non-TOKEN_ENDS in ttext, but add defects to the token's
+    defects list if we find non-ttext characters.  We also register defects for
+    *any* non-printables even though the RFC doesn't exclude all of them,
+    because we follow the spirit of RFC 5322.
+
+    """
+    m = _non_token_end_matcher(value)
+    if not m:
+        raise errors.HeaderParseError(
+            "expected ttext but found '{}'".format(value))
+    ttext = m.group()
+    value = value[len(ttext):]
+    ttext = ValueTerminal(ttext, 'ttext')
+    _validate_xtext(ttext)
+    return ttext, value
+
+def get_token(value):
+    """token = [CFWS] 1*ttext [CFWS]
+
+    The RFC equivalent of ttext is any US-ASCII chars except space, ctls, or
+    tspecials.  We also exclude tabs even though the RFC doesn't.
+
+    The RFC implies the CFWS but is not explicit about it in the BNF.
+
+    """
+    mtoken = Token()
+    if value and value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        mtoken.append(token)
+    if value and value[0] in TOKEN_ENDS:
+        raise errors.HeaderParseError(
+            "expected token but found '{}'".format(value))
+    token, value = get_ttext(value)
+    mtoken.append(token)
+    if value and value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        mtoken.append(token)
+    return mtoken, value
+
+def get_attrtext(value):
+    """attrtext = 1*(any non-ATTRIBUTE_ENDS character)
+
+    We allow any non-ATTRIBUTE_ENDS in attrtext, but add defects to the
+    token's defects list if we find non-attrtext characters.  We also register
+    defects for *any* non-printables even though the RFC doesn't exclude all of
+    them, because we follow the spirit of RFC 5322.
+
+    """
+    m = _non_attribute_end_matcher(value)
+    if not m:
+        raise errors.HeaderParseError(
+            "expected attrtext but found {!r}".format(value))
+    attrtext = m.group()
+    value = value[len(attrtext):]
+    attrtext = ValueTerminal(attrtext, 'attrtext')
+    _validate_xtext(attrtext)
+    return attrtext, value
+
+def get_attribute(value):
+    """ [CFWS] 1*attrtext [CFWS]
+
+    This version of the BNF makes the CFWS explicit, and as usual we use a
+    value terminal for the actual run of characters.  The RFC equivalent of
+    attrtext is the token characters, with the subtraction of '*', "'", and '%'.
+    We include tab in the excluded set just as we do for token.
+
+    """
+    attribute = Attribute()
+    if value and value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        attribute.append(token)
+    if value and value[0] in ATTRIBUTE_ENDS:
+        raise errors.HeaderParseError(
+            "expected token but found '{}'".format(value))
+    token, value = get_attrtext(value)
+    attribute.append(token)
+    if value and value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        attribute.append(token)
+    return attribute, value
+
+def get_extended_attrtext(value):
+    """attrtext = 1*(any non-ATTRIBUTE_ENDS character plus '%')
+
+    This is a special parsing routine so that we get a value that
+    includes % escapes as a single string (which we decode as a single
+    string later).
+
+    """
+    m = _non_extended_attribute_end_matcher(value)
+    if not m:
+        raise errors.HeaderParseError(
+            "expected extended attrtext but found {!r}".format(value))
+    attrtext = m.group()
+    value = value[len(attrtext):]
+    attrtext = ValueTerminal(attrtext, 'extended-attrtext')
+    _validate_xtext(attrtext)
+    return attrtext, value
+
+def get_extended_attribute(value):
+    """ [CFWS] 1*extended_attrtext [CFWS]
+
+    This is like the non-extended version except we allow % characters, so that
+    we can pick up an encoded value as a single string.
+
+    """
+    # XXX: should we have an ExtendedAttribute TokenList?
+    attribute = Attribute()
+    if value and value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        attribute.append(token)
+    if value and value[0] in EXTENDED_ATTRIBUTE_ENDS:
+        raise errors.HeaderParseError(
+            "expected token but found '{}'".format(value))
+    token, value = get_extended_attrtext(value)
+    attribute.append(token)
+    if value and value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        attribute.append(token)
+    return attribute, value
+
+def get_section(value):
+    """ '*' digits
+
+    The formal BNF is more complicated because leading 0s are not allowed.  We
+    check for that and add a defect.  We also assume no CFWS is allowed between
+    the '*' and the digits, though the RFC is not crystal clear on that.
+    The caller should already have dealt with leading CFWS.
+
+    """
+    section = Section()
+    if not value or value[0] != '*':
+        raise errors.HeaderParseError("Expected section but found {}".format(
+                                        value))
+    section.append(ValueTerminal('*', 'section-marker'))
+    value = value[1:]
+    if not value or not value[0].isdigit():
+        raise errors.HeaderParseError("Expected section number but "
+                                      "found {}".format(value))
+    digits = ''
+    while value and value[0].isdigit():
+        digits += value[0]
+        value = value[1:]
+    if digits[0] == '0' and digits != '0':
+        section.defects.append(errors.InvalidHeaderDefect(
+                "section number has an invalid leading 0"))
+    section.number = int(digits)
+    section.append(ValueTerminal(digits, 'digits'))
+    return section, value
+
+
+def get_value(value):
+    """ quoted-string / attribute
+
+    """
+    v = Value()
+    if not value:
+        raise errors.HeaderParseError("Expected value but found end of string")
+    leader = None
+    if value[0] in CFWS_LEADER:
+        leader, value = get_cfws(value)
+    if not value:
+        raise errors.HeaderParseError("Expected value but found "
+                                      "only {}".format(leader))
+    if value[0] == '"':
+        token, value = get_quoted_string(value)
+    else:
+        token, value = get_extended_attribute(value)
+    if leader is not None:
+        token[:0] = [leader]
+    v.append(token)
+    return v, value
+
+def get_parameter(value):
+    """ attribute [section] ["*"] [CFWS] "=" value
+
+    The CFWS is implied by the RFC but not made explicit in the BNF.  This
+    simplified form of the BNF from the RFC is made to conform with the RFC BNF
+    through some extra checks.  We do it this way because it makes both error
+    recovery and working with the resulting parse tree easier.
+    """
+    # It is possible CFWS would also be implicitly allowed between the section
+    # and the 'extended-attribute' marker (the '*') , but we've never seen that
+    # in the wild and we will therefore ignore the possibility.
+    param = Parameter()
+    token, value = get_attribute(value)
+    param.append(token)
+    if not value or value[0] == ';':
+        param.defects.append(errors.InvalidHeaderDefect("Parameter contains "
+            "name ({}) but no value".format(token)))
+        return param, value
+    if value[0] == '*':
+        try:
+            token, value = get_section(value)
+            param.sectioned = True
+            param.append(token)
+        except errors.HeaderParseError:
+            pass
+        if not value:
+            raise errors.HeaderParseError("Incomplete parameter")
+        if value[0] == '*':
+            param.append(ValueTerminal('*', 'extended-parameter-marker'))
+            value = value[1:]
+            param.extended = True
+    if value[0] != '=':
+        raise errors.HeaderParseError("Parameter not followed by '='")
+    param.append(ValueTerminal('=', 'parameter-separator'))
+    value = value[1:]
+    if value and value[0] in CFWS_LEADER:
+        token, value = get_cfws(value)
+        param.append(token)
+    remainder = None
+    appendto = param
+    if param.extended and value and value[0] == '"':
+        # Now for some serious hackery to handle the common invalid case of
+        # double quotes around an extended value.  We also accept (with defect)
+        # a value marked as encoded that isn't really.
+        qstring, remainder = get_quoted_string(value)
+        inner_value = qstring.stripped_value
+        semi_valid = False
+        if param.section_number == 0:
+            if inner_value and inner_value[0] == "'":
+                semi_valid = True
+            else:
+                token, rest = get_attrtext(inner_value)
+                if rest and rest[0] == "'":
+                    semi_valid = True
+        else:
+            try:
+                token, rest = get_extended_attrtext(inner_value)
+            except:
+                pass
+            else:
+                if not rest:
+                    semi_valid = True
+        if semi_valid:
+            param.defects.append(errors.InvalidHeaderDefect(
+                "Quoted string value for extended parameter is invalid"))
+            param.append(qstring)
+            for t in qstring:
+                if t.token_type == 'bare-quoted-string':
+                    t[:] = []
+                    appendto = t
+                    break
+            value = inner_value
+        else:
+            remainder = None
+            param.defects.append(errors.InvalidHeaderDefect(
+                "Parameter marked as extended but appears to have a "
+                "quoted string value that is non-encoded"))
+    if value and value[0] == "'":
+        token = None
+    else:
+        token, value = get_value(value)
+    if not param.extended or param.section_number > 0:
+        if not value or value[0] != "'":
+            appendto.append(token)
+            if remainder is not None:
+                assert not value, value
+                value = remainder
+            return param, value
+        param.defects.append(errors.InvalidHeaderDefect(
+            "Apparent initial-extended-value but attribute "
+            "was not marked as extended or was not initial section"))
+    if not value:
+        # Assume the charset/lang is missing and the token is the value.
+        param.defects.append(errors.InvalidHeaderDefect(
+            "Missing required charset/lang delimiters"))
+        appendto.append(token)
+        if remainder is None:
+            return param, value
+    else:
+        if token is not None:
+            for t in token:
+                if t.token_type == 'extended-attrtext':
+                    break
+            t.token_type == 'attrtext'
+            appendto.append(t)
+            param.charset = t.value
+        if value[0] != "'":
+            raise errors.HeaderParseError("Expected RFC2231 char/lang encoding "
+                                          "delimiter, but found {!r}".format(value))
+        appendto.append(ValueTerminal("'", 'RFC2231-delimiter'))
+        value = value[1:]
+        if value and value[0] != "'":
+            token, value = get_attrtext(value)
+            appendto.append(token)
+            param.lang = token.value
+            if not value or value[0] != "'":
+                raise errors.HeaderParseError("Expected RFC2231 char/lang encoding "
+                                  "delimiter, but found {}".format(value))
+        appendto.append(ValueTerminal("'", 'RFC2231-delimiter'))
+        value = value[1:]
+    if remainder is not None:
+        # Treat the rest of value as bare quoted string content.
+        v = Value()
+        while value:
+            if value[0] in WSP:
+                token, value = get_fws(value)
+            elif value[0] == '"':
+                token = ValueTerminal('"', 'DQUOTE')
+                value = value[1:]
+            else:
+                token, value = get_qcontent(value)
+            v.append(token)
+        token = v
+    else:
+        token, value = get_value(value)
+    appendto.append(token)
+    if remainder is not None:
+        assert not value, value
+        value = remainder
+    return param, value
+
+def parse_mime_parameters(value):
+    """ parameter *( ";" parameter )
+
+    That BNF is meant to indicate this routine should only be called after
+    finding and handling the leading ';'.  There is no corresponding rule in
+    the formal RFC grammar, but it is more convenient for us for the set of
+    parameters to be treated as its own TokenList.
+
+    This is 'parse' routine because it consumes the remaining value, but it
+    would never be called to parse a full header.  Instead it is called to
+    parse everything after the non-parameter value of a specific MIME header.
+
+    """
+    mime_parameters = MimeParameters()
+    while value:
+        try:
+            token, value = get_parameter(value)
+            mime_parameters.append(token)
+        except errors.HeaderParseError:
+            leader = None
+            if value[0] in CFWS_LEADER:
+                leader, value = get_cfws(value)
+            if not value:
+                mime_parameters.append(leader)
+                return mime_parameters
+            if value[0] == ';':
+                if leader is not None:
+                    mime_parameters.append(leader)
+                mime_parameters.defects.append(errors.InvalidHeaderDefect(
+                    "parameter entry with no content"))
+            else:
+                token, value = get_invalid_parameter(value)
+                if leader:
+                    token[:0] = [leader]
+                mime_parameters.append(token)
+                mime_parameters.defects.append(errors.InvalidHeaderDefect(
+                    "invalid parameter {!r}".format(token)))
+        if value and value[0] != ';':
+            # Junk after the otherwise valid parameter.  Mark it as
+            # invalid, but it will have a value.
+            param = mime_parameters[-1]
+            param.token_type = 'invalid-parameter'
+            token, value = get_invalid_parameter(value)
+            param.extend(token)
+            mime_parameters.defects.append(errors.InvalidHeaderDefect(
+                "parameter with invalid trailing text {!r}".format(token)))
+        if value:
+            # Must be a ';' at this point.
+            mime_parameters.append(ValueTerminal(';', 'parameter-separator'))
+            value = value[1:]
+    return mime_parameters
+
+def _find_mime_parameters(tokenlist, value):
+    """Do our best to find the parameters in an invalid MIME header
+
+    """
+    while value and value[0] != ';':
+        if value[0] in PHRASE_ENDS:
+            tokenlist.append(ValueTerminal(value[0], 'misplaced-special'))
+            value = value[1:]
+        else:
+            token, value = get_phrase(value)
+            tokenlist.append(token)
+    if not value:
+        return
+    tokenlist.append(ValueTerminal(';', 'parameter-separator'))
+    tokenlist.append(parse_mime_parameters(value[1:]))
+
+def parse_content_type_header(value):
+    """ maintype "/" subtype *( ";" parameter )
+
+    The maintype and substype are tokens.  Theoretically they could
+    be checked against the official IANA list + x-token, but we
+    don't do that.
+    """
+    ctype = ContentType()
+    if not value:
+        ctype.defects.append(errors.HeaderMissingRequiredValue(
+            "Missing content type specification"))
+        return ctype
+    try:
+        token, value = get_token(value)
+    except errors.HeaderParseError:
+        ctype.defects.append(errors.InvalidHeaderDefect(
+            "Expected content maintype but found {!r}".format(value)))
+        _find_mime_parameters(ctype, value)
+        return ctype
+    ctype.append(token)
+    # XXX: If we really want to follow the formal grammar we should make
+    # mantype and subtype specialized TokenLists here.  Probably not worth it.
+    if not value or value[0] != '/':
+        ctype.defects.append(errors.InvalidHeaderDefect(
+            "Invalid content type"))
+        if value:
+            _find_mime_parameters(ctype, value)
+        return ctype
+    ctype.maintype = token.value.strip().lower()
+    ctype.append(ValueTerminal('/', 'content-type-separator'))
+    value = value[1:]
+    try:
+        token, value = get_token(value)
+    except errors.HeaderParseError:
+        ctype.defects.append(errors.InvalidHeaderDefect(
+            "Expected content subtype but found {!r}".format(value)))
+        _find_mime_parameters(ctype, value)
+        return ctype
+    ctype.append(token)
+    ctype.subtype = token.value.strip().lower()
+    if not value:
+        return ctype
+    if value[0] != ';':
+        ctype.defects.append(errors.InvalidHeaderDefect(
+            "Only parameters are valid after content type, but "
+            "found {!r}".format(value)))
+        # The RFC requires that a syntactically invalid content-type be treated
+        # as text/plain.  Perhaps we should postel this, but we should probably
+        # only do that if we were checking the subtype value against IANA.
+        del ctype.maintype, ctype.subtype
+        _find_mime_parameters(ctype, value)
+        return ctype
+    ctype.append(ValueTerminal(';', 'parameter-separator'))
+    ctype.append(parse_mime_parameters(value[1:]))
+    return ctype
+
+def parse_content_disposition_header(value):
+    """ disposition-type *( ";" parameter )
+
+    """
+    disp_header = ContentDisposition()
+    if not value:
+        disp_header.defects.append(errors.HeaderMissingRequiredValue(
+            "Missing content disposition"))
+        return disp_header
+    try:
+        token, value = get_token(value)
+    except errors.HeaderParseError:
+        disp_header.defects.append(errors.InvalidHeaderDefect(
+            "Expected content disposition but found {!r}".format(value)))
+        _find_mime_parameters(disp_header, value)
+        return disp_header
+    disp_header.append(token)
+    disp_header.content_disposition = token.value.strip().lower()
+    if not value:
+        return disp_header
+    if value[0] != ';':
+        disp_header.defects.append(errors.InvalidHeaderDefect(
+            "Only parameters are valid after content disposition, but "
+            "found {!r}".format(value)))
+        _find_mime_parameters(disp_header, value)
+        return disp_header
+    disp_header.append(ValueTerminal(';', 'parameter-separator'))
+    disp_header.append(parse_mime_parameters(value[1:]))
+    return disp_header
+
+def parse_content_transfer_encoding_header(value):
+    """ mechanism
+
+    """
+    # We should probably validate the values, since the list is fixed.
+    cte_header = ContentTransferEncoding()
+    if not value:
+        cte_header.defects.append(errors.HeaderMissingRequiredValue(
+            "Missing content transfer encoding"))
+        return cte_header
+    try:
+        token, value = get_token(value)
+    except errors.HeaderParseError:
+        cte_header.defects.append(errors.InvalidHeaderDefect(
+            "Expected content transfer encoding but found {!r}".format(value)))
+    else:
+        cte_header.append(token)
+        cte_header.cte = token.value.strip().lower()
+    if not value:
+        return cte_header
+    while value:
+        cte_header.defects.append(errors.InvalidHeaderDefect(
+            "Extra text after content transfer encoding"))
+        if value[0] in PHRASE_ENDS:
+            cte_header.append(ValueTerminal(value[0], 'misplaced-special'))
+            value = value[1:]
+        else:
+            token, value = get_phrase(value)
+            cte_header.append(token)
+    return cte_header
+
+
+#
+# Header folding
+#
+# Header folding is complex, with lots of rules and corner cases.  The
+# following code does its best to obey the rules and handle the corner
+# cases, but you can be sure there are few bugs:)
+#
+# This folder generally canonicalizes as it goes, preferring the stringified
+# version of each token.  The tokens contain information that supports the
+# folder, including which tokens can be encoded in which ways.
+#
+# Folded text is accumulated in a simple list of strings ('lines'), each
+# one of which should be less than policy.max_line_length ('maxlen').
+#
+
+def _steal_trailing_WSP_if_exists(lines):
+    wsp = ''
+    if lines and lines[-1] and lines[-1][-1] in WSP:
+        wsp = lines[-1][-1]
+        lines[-1] = lines[-1][:-1]
+    return wsp
+
+def _refold_parse_tree(parse_tree, *, policy):
+    """Return string of contents of parse_tree folded according to RFC rules.
+
+    """
+    # max_line_length 0/None means no limit, ie: infinitely long.
+    maxlen = policy.max_line_length or sys.maxsize
+    encoding = 'utf-8' if policy.utf8 else 'us-ascii'
+    lines = ['']
+    last_ew = None
+    last_charset = None
+    wrap_as_ew_blocked = 0
+    want_encoding = False
+    end_ew_not_allowed = Terminal('', 'wrap_as_ew_blocked')
+    parts = list(parse_tree)
+    while parts:
+        part = parts.pop(0)
+        if part is end_ew_not_allowed:
+            wrap_as_ew_blocked -= 1
+            continue
+        tstr = str(part)
+        if part.token_type == 'ptext' and set(tstr) & SPECIALS:
+            # Encode if tstr contains special characters.
+            want_encoding = True
+        try:
+            tstr.encode(encoding)
+            charset = encoding
+        except UnicodeEncodeError:
+            if any(isinstance(x, errors.UndecodableBytesDefect)
+                   for x in part.all_defects):
+                charset = 'unknown-8bit'
+            else:
+                # If policy.utf8 is false this should really be taken from a
+                # 'charset' property on the policy.
+                charset = 'utf-8'
+            want_encoding = True
+        if part.token_type == 'mime-parameters':
+            # Mime parameter folding (using RFC2231) is extra special.
+            _fold_mime_parameters(part, lines, maxlen, encoding)
+            continue
+        if want_encoding and not wrap_as_ew_blocked:
+            if not part.as_ew_allowed:
+                want_encoding = False
+                last_ew = None
+                if part.syntactic_break:
+                    encoded_part = part.fold(policy=policy)[:-len(policy.linesep)]
+                    if policy.linesep not in encoded_part:
+                        # It fits on a single line
+                        if len(encoded_part) > maxlen - len(lines[-1]):
+                            # But not on this one, so start a new one.
+                            newline = _steal_trailing_WSP_if_exists(lines)
+                            # XXX what if encoded_part has no leading FWS?
+                            lines.append(newline)
+                        lines[-1] += encoded_part
+                        continue
+                # Either this is not a major syntactic break, so we don't
+                # want it on a line by itself even if it fits, or it
+                # doesn't fit on a line by itself.  Either way, fall through
+                # to unpacking the subparts and wrapping them.
+            if not hasattr(part, 'encode'):
+                # It's not a Terminal, do each piece individually.
+                parts = list(part) + parts
+            else:
+                # It's a terminal, wrap it as an encoded word, possibly
+                # combining it with previously encoded words if allowed.
+                if (last_ew is not None and
+                    charset != last_charset and
+                    (last_charset == 'unknown-8bit' or
+                     last_charset == 'utf-8' and charset != 'us-ascii')):
+                    last_ew = None
+                last_ew = _fold_as_ew(tstr, lines, maxlen, last_ew,
+                                      part.ew_combine_allowed, charset)
+                last_charset = charset
+            want_encoding = False
+            continue
+        if len(tstr) <= maxlen - len(lines[-1]):
+            lines[-1] += tstr
+            continue
+        # This part is too long to fit.  The RFC wants us to break at
+        # "major syntactic breaks", so unless we don't consider this
+        # to be one, check if it will fit on the next line by itself.
+        if (part.syntactic_break and
+                len(tstr) + 1 <= maxlen):
+            newline = _steal_trailing_WSP_if_exists(lines)
+            if newline or part.startswith_fws():
+                lines.append(newline + tstr)
+                last_ew = None
+                continue
+        if not hasattr(part, 'encode'):
+            # It's not a terminal, try folding the subparts.
+            newparts = list(part)
+            if not part.as_ew_allowed:
+                wrap_as_ew_blocked += 1
+                newparts.append(end_ew_not_allowed)
+            parts = newparts + parts
+            continue
+        if part.as_ew_allowed and not wrap_as_ew_blocked:
+            # It doesn't need CTE encoding, but encode it anyway so we can
+            # wrap it.
+            parts.insert(0, part)
+            want_encoding = True
+            continue
+        # We can't figure out how to wrap, it, so give up.
+        newline = _steal_trailing_WSP_if_exists(lines)
+        if newline or part.startswith_fws():
+            lines.append(newline + tstr)
+        else:
+            # We can't fold it onto the next line either...
+            lines[-1] += tstr
+    return policy.linesep.join(lines) + policy.linesep
+
+def _fold_as_ew(to_encode, lines, maxlen, last_ew, ew_combine_allowed, charset):
+    """Fold string to_encode into lines as encoded word, combining if allowed.
+    Return the new value for last_ew, or None if ew_combine_allowed is False.
+
+    If there is already an encoded word in the last line of lines (indicated by
+    a non-None value for last_ew) and ew_combine_allowed is true, decode the
+    existing ew, combine it with to_encode, and re-encode.  Otherwise, encode
+    to_encode.  In either case, split to_encode as necessary so that the
+    encoded segments fit within maxlen.
+
+    """
+    if last_ew is not None and ew_combine_allowed:
+        to_encode = str(
+            get_unstructured(lines[-1][last_ew:] + to_encode))
+        lines[-1] = lines[-1][:last_ew]
+    if to_encode[0] in WSP:
+        # We're joining this to non-encoded text, so don't encode
+        # the leading blank.
+        leading_wsp = to_encode[0]
+        to_encode = to_encode[1:]
+        if (len(lines[-1]) == maxlen):
+            lines.append(_steal_trailing_WSP_if_exists(lines))
+        lines[-1] += leading_wsp
+    trailing_wsp = ''
+    if to_encode[-1] in WSP:
+        # Likewise for the trailing space.
+        trailing_wsp = to_encode[-1]
+        to_encode = to_encode[:-1]
+    new_last_ew = len(lines[-1]) if last_ew is None else last_ew
+
+    encode_as = 'utf-8' if charset == 'us-ascii' else charset
+
+    # The RFC2047 chrome takes up 7 characters plus the length
+    # of the charset name.
+    chrome_len = len(encode_as) + 7
+
+    if (chrome_len + 1) >= maxlen:
+        raise errors.HeaderParseError(
+            "max_line_length is too small to fit an encoded word")
+
+    while to_encode:
+        remaining_space = maxlen - len(lines[-1])
+        text_space = remaining_space - chrome_len
+        if text_space <= 0:
+            lines.append(' ')
+            continue
+
+        to_encode_word = to_encode[:text_space]
+        encoded_word = _ew.encode(to_encode_word, charset=encode_as)
+        excess = len(encoded_word) - remaining_space
+        while excess > 0:
+            # Since the chunk to encode is guaranteed to fit into less than 100 characters,
+            # shrinking it by one at a time shouldn't take long.
+            to_encode_word = to_encode_word[:-1]
+            encoded_word = _ew.encode(to_encode_word, charset=encode_as)
+            excess = len(encoded_word) - remaining_space
+        lines[-1] += encoded_word
+        to_encode = to_encode[len(to_encode_word):]
+
+        if to_encode:
+            lines.append(' ')
+            new_last_ew = len(lines[-1])
+    lines[-1] += trailing_wsp
+    return new_last_ew if ew_combine_allowed else None
+
+def _fold_mime_parameters(part, lines, maxlen, encoding):
+    """Fold TokenList 'part' into the 'lines' list as mime parameters.
+
+    Using the decoded list of parameters and values, format them according to
+    the RFC rules, including using RFC2231 encoding if the value cannot be
+    expressed in 'encoding' and/or the parameter+value is too long to fit
+    within 'maxlen'.
+
+    """
+    # Special case for RFC2231 encoding: start from decoded values and use
+    # RFC2231 encoding iff needed.
+    #
+    # Note that the 1 and 2s being added to the length calculations are
+    # accounting for the possibly-needed spaces and semicolons we'll be adding.
+    #
+    for name, value in part.params:
+        # XXX What if this ';' puts us over maxlen the first time through the
+        # loop?  We should split the header value onto a newline in that case,
+        # but to do that we need to recognize the need earlier or reparse the
+        # header, so I'm going to ignore that bug for now.  It'll only put us
+        # one character over.
+        if not lines[-1].rstrip().endswith(';'):
+            lines[-1] += ';'
+        charset = encoding
+        error_handler = 'strict'
+        try:
+            value.encode(encoding)
+            encoding_required = False
+        except UnicodeEncodeError:
+            encoding_required = True
+            if utils._has_surrogates(value):
+                charset = 'unknown-8bit'
+                error_handler = 'surrogateescape'
+            else:
+                charset = 'utf-8'
+        if encoding_required:
+            encoded_value = urllib.parse.quote(
+                value, safe='', errors=error_handler)
+            tstr = "{}*={}''{}".format(name, charset, encoded_value)
+        else:
+            tstr = '{}={}'.format(name, quote_string(value))
+        if len(lines[-1]) + len(tstr) + 1 < maxlen:
+            lines[-1] = lines[-1] + ' ' + tstr
+            continue
+        elif len(tstr) + 2 <= maxlen:
+            lines.append(' ' + tstr)
+            continue
+        # We need multiple sections.  We are allowed to mix encoded and
+        # non-encoded sections, but we aren't going to.  We'll encode them all.
+        section = 0
+        extra_chrome = charset + "''"
+        while value:
+            chrome_len = len(name) + len(str(section)) + 3 + len(extra_chrome)
+            if maxlen <= chrome_len + 3:
+                # We need room for the leading blank, the trailing semicolon,
+                # and at least one character of the value.  If we don't
+                # have that, we'd be stuck, so in that case fall back to
+                # the RFC standard width.
+                maxlen = 78
+            splitpoint = maxchars = maxlen - chrome_len - 2
+            while True:
+                partial = value[:splitpoint]
+                encoded_value = urllib.parse.quote(
+                    partial, safe='', errors=error_handler)
+                if len(encoded_value) <= maxchars:
+                    break
+                splitpoint -= 1
+            lines.append(" {}*{}*={}{}".format(
+                name, section, extra_chrome, encoded_value))
+            extra_chrome = ''
+            section += 1
+            value = value[splitpoint:]
+            if value:
+                lines[-1] += ';'