timed global protocol Calc(role A, role C, role S)
{
    element_1(int) from C to S within [0;1] using a and resetting ();
    element_2(int) from C to S within [0;1] using a and resetting ();

    choice at C
    {
        sum(int) from S to C within [0;1] using a and resetting ();
    }
    or
    {
        diff(int) from S to C within [0;1] using a and resetting ();
    }
}