defproc threeway($e1, $e2, $note="")
{
    $e1->$e2: SYN;
    if $note then {
        $e1<-*$e2: SYNACK;
        note [label=$note];
    } else {
        $e1<-$e2: SYNACK;
        $e1->$e2: ACK;
    };
};

replay threeway(a, b);
replay threeway(b, c, "Lost, so we fail.");