element(_, [], []).
element(X, [X|S], R) :- element(X, S, R), !.
element(X, [C|S], [C|R]) :- element(X, S, R).

?- element(2, [1,2,3], [1,3]).
true % good

?- element(3, [1,2,3], [1,3]).
false % good

?- element(2, [1,2,3], R).
R = [1,3] % good

?- element(2, R, [1,3]).
infinite recursion % bad, must have R = [2, 1, 3]