Often in SPMD programming it is necessary to restrict execution of a block of code to processors in a particular group p. Our language provides a short way of writing this construct
on(p) {
...
}
The language incorporates a formal idea of an active process group (APG). At any point of execution some process group is singled out as the APG. An on(p) construct specifically changes the value of the APG to p. On exit from the construct, the APG is restored to its value on entry.