code: def join_lines(lines_enum):
      # type: (ReqFileLines) -> ReqFileLines
      
      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)
docs: Joins a line ending in '\' with the previous line (except when following
      comments).  The joined line takes on the index of the first line.