check(Rel, A, B):-

    Goal =.. [Rel,A,B],

    call(Goal).

merge(_,[],[],[]).


merge(Rel,[H1|T1],[H2|T2], [H1,H2|L]):-
    check(Rel, H1, H2),
    merge(Rel, T1, T2, L).

merge(Rel,[H1|T1],[H2|T2], L):-
    \+ check(Rel, H1, H2),
    merge(Rel, T1, T2, L).

merge2(_,[],[],[]).
merge2(Rel,[H1|T1],[H2|T2], L):-
    check(Rel, H1, H2),
    append([H1|_],[H2|_],L),
    merge2(Rel,T1,T2,L).


merge2(Rel,[H1|T1],[H2|T2], L):-
    \+ check(Rel, H1, H2),
    merge2(Rel,T1,T2,L).