global protocol ThreeBuyers(role A, role C, role S)
{
    empty1(int) from A to S;
    empty2(int) from S to A;
    empty3(int) from S to C;
    empty4(int) from A to C;

    choice at C
    {
        ok(int) from C to A;
        ok(int) from C to S;
        empty5(int) from S to C;
    }
    or
    {
        quit() from C to A;
        quit() from C to S;
    }
}