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%
%  Check whether all elements of a set S are pairs, 
%  i.e., they have the form [x1,x2], for any x1 and x2
%
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all_pairs(S) :- 
         forall(X in S, exists([X1,X2], X = [X1,X2])).


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% Sample goal:
%
% {log}=> all_pairs({[peter,ann],[tom,mary],[john,ann]}).
%
% yes