Prolog - pairs that are not related

parent(kresimir, jasna).
parent(kresimir, darko).
parent(vesna, darko).
parent(darko, vigor).
parent(darko, goran).
parent(vigor, ruzica).
parent(vigor, snjezana).
parent(mario, nenad).

ancestor(X, Y) :-
    parent(X, Y).

ancestor(X, Y) :-
    parent(X, Z),
    ancestor(Z, Y).


related(X, Y) :-
    parent(X,Y);
    parent(Y,X).
related(X, Y) :-
    X \= Y,
    ancestor(Z, X),
    ancestor(Z, Y).

This is my prolog file, what i need is to find开发者_运维问答 all pairs of people that are not related, and i have no idea how.

You need a way get all the people first.

person(X) :- parent(X, _).
person(X) :- parent(_, X).

Then use setof/3 to get a list of people without duplicates:

setof(X, person(X), People)

and collect all pairs from that:

pair_of_people(X,Y) :-
    setof(P, person(P), People),
    member(X, People),
    member(Y, People),
    X \= Y.

This, by the way, produces "mirror" pairs: both (X,Y) and (Y,X) for all X and Y. You can do better with an adapted version of member that many Prologs support. Check out your Prolog manual.

Finally, filter out the pairs that are related:

unrelated(X,Y) :-
    pair_of_people(X,Y),
    not(related(X,Y)).