%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
%  Check whether an element X belongs to the difference 
%  between two sets, Set1 and Set2 (i.e., X in Set1\Set2)
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


in_difference(X,Set1,Set2) :-
            set(Set1) & set(Set2) &
            X in Set1 & 
            X nin Set2.	


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
% Sample goals:
%
% {log}=> in_difference(X,{1,2},{1,3}).  
% 
% X = 2
%
% {log}=> in_difference(X,S1,{1,3}).      
%
% S1 = {X/N} 
% Constraint: set(N), X neq 1, X neq 3