docs: def get_index_line(self,lnum):
      """ Take the 1-indexed line number and return its index information"""
      if lnum < 1:
        sys.stderr.write("ERROR: line number should be greater than zero\n")
        sys.exit()
      elif lnum > len(self._lines):
        sys.stderr.write("ERROR: too far this line nuber is not in index\n")
        sys.exit()
      return self._lines[lnum-1]