CLP(Tree)

• freeze(X,Goal): This is equivalent to once(Goal), except that the evaluation is delayed until X becomes a non-variable term. The predicate is defined as follows:

       freeze(X,Goal),var(X),{ins(X)} => true.
       freeze(X,Goal) => call(Goal).
  

  If X is a variable, the agent freeze(X,Goal) is delayed. When X is bound, an event ins(X) is posted automatically, which will in turn activate the agent freeze(X,Goal). If X is not a variable, then the second rule will rewrite freeze(X,Goal) into call(Goal). Note that, since agents can never succeed more than once, Goal in freeze(X,Goal) cannot return multiple solutions. This is a big difference from the freeze predicate in Prolog-II.

• dif(T1,T2): The two terms T1 and T2 are different. If T1 and T2 are not arithmetic expressions, the constraint can be written as T1 #\= T2.