Example:

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The following example shows how to attach a finite-domain to a variable:

       create_fd_variable(X,D):-
           put_attr(X,fd,D),
           check_value(X,D).
      
       check_value(X,D),var(X),{ins(X)} => true.
       check_value(X,D) => member(X,D).

The agent check_value(X,D) is activated to check whether the value is in the domain when X is instantiated. This predicate can be defined equivalently as follows:

       create_fd_variable(X,D):-
           put_attr_with_hook(X,fd,D).
      
       attr_unify_hook(X,fd,D):-member(X,D).

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