Execute Goal when Condition is satisfied. I.e., Goal is executed
as by call/1 if Condition is true when when/2 is called.
Otherwise Goal is delayed until Condition becomes true.
Condition is one of the following:
nonvar(X)ground(X)- ?=(X,Y)
- (Cond1,Cond2)
- (Cond2;Cond2)
For example (note the order a and b are written):
?- when(nonvar(X), writeln(a)), writeln(b), X = x.
b
a
X = x