:- initialization(run).
 
/* A + B = C */
abc(A,B,C) :- nonvar(A), nonvar(B), write('A+B '), !, C is A+B.
abc(A,B,C) :- nonvar(A), nonvar(C), write('C-A '), !, B is C-A.
abc(A,B,C) :- nonvar(B), nonvar(C), write('C-B '), !, A is C-B.
/* no idea how to solve */
abc(A,B,C) :- write('figvam '), fail.
 
writes([]) :- write('\n').
writes([X|Xs]) :- write(X), writes(Xs).
 
test(A,B,C) :-
	writes(['start...']),
	abc(A,B,C),
	writes(['A=',A, ' B=',B, ' C=',C]);
	writes(['failed.']).
 
run :-
	test(1,4,_),
	test(1,_,5),
	test(_,4,5),
	test(1,4,5),
	test(1,2,10),
	test(_,_,10),
	test(_,10,_),
	test(10,_,_).

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