code: def get_mcs_index(mcs):
      
      answers = list()
      for mcs_bit in range(77):
          mcs_octet = int(mcs_bit / 8)
          mcs_rate_bit = 1 << mcs_bit % 8
          mcs_rate_idx_set = not not (mcs[mcs_octet] & mcs_rate_bit)
          if not mcs_rate_idx_set:
              continue
          answers.append(mcs_bit)
      return answers
docs: http://git.kernel.org/cgit/linux/kernel/git/jberg/iw.git/tree/util.c?id=v3.17#n453.
  
      Positional arguments:
      mcs -- bytearray
  
      Returns:
      List.