is_member(V, [V | _]) :- !.
is_member(V, [_ | T]) :-
    is_member(V, T).


delete_element([], _V, []) :- !.
delete_element([V | T], V, NT) :-
    !,
    delete_element(T, V, NT).
delete_element([V1 | T], V, [V1 | NT]) :-
    delete_element(T, V, NT).


delete_by_set(R, [], R) :- !.
delete_by_set(L, [VS | TS], R) :-
    delete_element(L, VS, TLR),
    delete_by_set(TLR, TS, R).