Remember, the parallel composition of condition processes does not need to specify a disjoint situation as in an IF statement. So we can write
Z<a,b> = (GTR<a,b> ; P | GTR<b,a> ; Q) .
In the case that z=b, neither condition process will terminate in STOP and hence neither P nor Q will be started.
-- (c) Fordham University Robotics and Computer Vision