chan send1, rec1;
chan send2, rec2;
chan e1, er1, e2, er2;
chan b1, br1, b2, br2;
chan c1, c2;
chan ack1, ack2;MACchan send, rec, b, e, c, br, eraddoneghtransferMacddonegjkltransfer1transfer2MAC1 := MAC( send1, rec1, b1, e1, c1, br1, er1 );
MAC2 := MAC( send2, rec2, b2, e2, c2, br2, er2 );system
MAC1, MAC2, M;