def mutate_stream (instream, outstream):
    """Python 3 compat note: we're assuming `stream` gives bytes not unicode."""

    chunk = None
    key = None
    data = None
    misclines = []

    for fullline in instream:
        line = fullline.split ('#', 1)[0]

        m = sectionre.match (line)
        if m is not None:
            # New chunk
            if chunk is not None:
                if key is not None:
                    chunk.data.set_one (key, data.strip ().decode ('utf8'))
                    key = data = None
                yield chunk
                chunk.emit (outstream)

            chunk = FileChunk ()
            for miscline in misclines:
                chunk._addLine (miscline, None)
            misclines = []
            chunk.data.section = m.group (1)
            chunk._addLine (fullline, None)
            continue

        if len (line.strip ()) == 0:
            if key is not None:
                chunk.data.set_one (key, data.strip ().decode ('utf8'))
                key = data = None
            if chunk is not None:
                chunk._addLine (fullline, None)
            else:
                misclines.append (fullline)
            continue

        m = escre.match (fullline)
        if m is not None:
            if chunk is None:
                raise InifileError ('key seen without section!')
            if key is not None:
                chunk.data.set_one (key, data.strip ().decode ('utf8'))
            key = m.group (1)
            data = m.group (2).replace (r'\"', '"').replace (r'\n', '\n').replace (r'\\', '\\')
            chunk.data.set_one (key, data.decode ('utf8'))
            chunk._addLine (fullline, key)
            key = data = None
            continue

        m = keyre.match (line)
        if m is not None:
            if chunk is None:
                raise InifileError ('key seen without section!')
            if key is not None:
                chunk.data.set_one (key, data.strip ().decode ('utf8'))
            key = m.group (1)
            data = m.group (2)
            if not data[-1].isspace ():
                data += ' '
            chunk._addLine (fullline, key)
            continue

        if line[0].isspace () and key is not None:
            data += line.strip () + ' '
            chunk._addLine (fullline, key)
            continue

        raise InifileError ('unparsable line: ' + line[:-1])

    if chunk is not None:
        if key is not None:
            chunk.data.set_one (key, data.strip ().decode ('utf8'))
        yield chunk
        chunk.emit (outstream)