Joins a line ending in '\' with the previous line (except when following
    comments).  The joined line takes on the index of the first line.

def join_lines(lines_enum):
    # type: (ReqFileLines) -> ReqFileLines
    """Joins a line ending in '\' with the previous line (except when following
    comments).  The joined line takes on the index of the first line.
    """
    primary_line_number = None
    new_line = []  # type: List[Text]
    for line_number, line in lines_enum:
        if not line.endswith('\\') or COMMENT_RE.match(line):
            if COMMENT_RE.match(line):
                # this ensures comments are always matched later
                line = ' ' + line
            if new_line:
                new_line.append(line)
                yield primary_line_number, ''.join(new_line)
                new_line = []
            else:
                yield line_number, line
        else:
            if not new_line:
                primary_line_number = line_number
            new_line.append(line.strip('\\'))

    # last line contains \
    if new_line:
        yield primary_line_number, ''.join(new_line)