docs: def guess_payload_class(self, payload):
          """
          ISOTP encodes the frame type in the first nibble of a frame.
          """
          t = (orb(payload[0]) & 0xf0) >> 4
          if t == 0:
              return ISOTP_SF
          elif t == 1:
              return ISOTP_FF
          elif t == 2:
              return ISOTP_CF
          else:
              return ISOTP_FC