code: def _strand_flag(data):
      
      strand_flag = {"unstranded": "0",
                     "firststrand": "2",
                     "secondstrand": "1"}
      stranded = dd.get_strandedness(data)
  
      assert stranded in strand_flag, ("%s is not a valid strandedness value. "
                                       "Valid values are 'firststrand', 'secondstrand', "
                                       "and 'unstranded")
      return strand_flag[stranded]
docs: 0: unstranded 1: stranded 2: reverse stranded