If your program is running a loop and you want it to exit the loop without finishing it, you can use the break command. For example, you can define a program
testbreak(a,b) := {
local r;
while (true) {
if (b == 0) {break;}
r := irem(a,b);
a := b;
b := r;
}
return a;
}
If you then enter
it will return
since the while loop is interrupted when b is 0 and a is 4.