(bitpacket PacketA
  ("PACKET_A") (Afield 8))

(bitpacket PacketB
  ("PACKET_B") (Bfield 16))
  
(bitmatch somedata
  ((PacketA) -> Afield)
  ((PacketB) -> Bfield)
  (else -> 'fail))
  
(define (make-PacketA Afield)
 (bitconstruct PacketA Afield))