1 Action = Packet +
2 datatype 'm action = S_msg ('m) | R_msg ('m)
3 | S_pkt ('m packet) | R_pkt ('m packet)
4 | S_ack (bool) | R_ack (bool)
5 | C_m_s | C_m_r | C_r_s | C_r_r ('m)
6 end