folding algorithm is simple in principle but
        # complex in practice.  Lines may be folded any place where "folding
        # white space" appears by inserting a linesep character in front of the
        # FWS.  The complication is that not all spaces or tabs qualify as FWS,
        # and we are also supposed to prefer to break at "higher level
        # syntactic breaks".  We can't do either of these without intimate
        # knowledge of the structure of structured headers, which we don't have
        # here.  So the best we can do here is prefer to break at the specified
        # splitchars, and hope that we don't choose any spaces or tabs that
        # aren't legal FWS.  (This is at least better than the old algorithm,
        # where we would sometimes *introduce* FWS after a splitchar, or the
        # algorithm before that, where we would turn all white space runs into
        # single spaces or tabs.)
        parts = re.split("(["+FWS+"]+)", fws+string)
        if parts[0]:
            parts[:0] = ['']
        else:
            parts.pop(0)
        for fws, part in zip(*[iter(parts)]*2):
            self._append_chunk(fws, part)

    def _append_chunk(self, fws, string):
        self._current_line.push(fws, string)
        if len(self._current_line) > self._maxlen:
            # Find the best split point, working backward from the end.
            # There might be none, on a long first line.
            for ch in self._splitchars:
                for i in range(self._current_line.part_count()-1, 0, -1):
                    if ch.isspace():
                        fws = self._current_line[i][0]
                        if fws and fws[0]==ch:
                            break
                    prevpart = self._current_line[i-1][1]
                    if prevpart and prevpart[-1]==ch:
                        break
                else:
                    continue
                break
            else:
                fws, part = self._current_line.pop()
                if self._current_line._initial_size > 0:
                    # There will be a header, so leave it on a line by itself.
                    self.newline()
                    if not fws:
                        # We don't use continuation_ws here because the whitespace
                        # after a header should always be a space.
                        fws = ' '
                self._current_line.push(fws, part)
                return
            remainder = self._current_line.pop_from(i)
            self._lines.append(str(self._current_line))
            self._current_line.reset(remainder)


class _Accumulator(list):

    def __init__(self, initial_size=0):
        self._initial_size = initial_size
        super().__init__()

    def push(self, fws, string):
        self.append((fws, string))

    def pop_from(self, i=0):
        popped = self[i:]
        self[i:] = []
        return popped

    def pop(self):
        if self.part_count()==0:
            return ('', '')
        return super().pop()

    def __len__(self):
        return sum((len(fws)+len(part) for fws, part in self),
                   self._initial_size)

    def __str__(self):
        return EMPTYSTRING.join((EMPTYSTRING.join((fws, part))
                                for fws, part in self))

    def reset(self, startval=None):
        if startval is None:
            startval = []
        self[:] = startval
        self._initial_size = 0

    def is_onlyws(self):
        return self._initial_size==0 and (not self or str(self).isspace())

    def part_count(self):
        return super().__len__()
