code: def check_ab(ab, verb):
      r
      try:
          ab = int(ab)
      except VariableCatch:
          print('* ERROR   :: <ab> must be an integer')
          raise
      pab = [11, 12, 13, 14, 15, 16, 21, 22, 23, 24, 25, 26,
             31, 32, 33, 34, 35, 36, 41, 42, 43, 44, 45, 46,
             51, 52, 53, 54, 55, 56, 61, 62, 63, 64, 65, 66]
      if ab not in pab:
          print('* ERROR   :: <ab> must be one of: ' + str(pab) + ';' +
                ' <ab> provided: ' + str(ab))
          raise ValueError('ab')
      if verb > 2:
          print("   Input ab        : ", ab)
      msrc = ab % 10 > 3
      mrec = ab // 10 > 3
      if mrec:
          if msrc:
              ab_calc = ab - 33
          else:
              ab_calc = ab % 10*10 + ab // 10
      else:
          ab_calc = ab
      if verb > 2:
          if ab in [36, 63]:
              print("\n>  <ab> IS "+str(ab)+" WHICH IS ZERO; returning")
          else:
              print("   Calculated ab   : ", ab_calc)
      return ab_calc, msrc, mrec