father(paul, john);
father(john, george);
father(tom, mary);
father(henk, martha);
mother(mary, john);
mother(martha, george);

parent(X, Y) :- father(X, Y);
parent(X, Y) :- mother(X, Y);

grandfather(X) :- parent(Y, X), father(Z, Y), Z;

ancient(X, Y) :- parent(X, Y);
ancient(X, Y) :- parent(X, Z), ancient(Z, Y);

allGf()  :- 
        Y = grandfather(X), 
        write(Y +" is grandfather of "+X+nl()), 
        fail;

allAnc()  :- 
        ancient(X, Y), 
        write(X +" is ancient of "+Y+nl()), 
        fail;



























gfd(X) :-
    (father(Y,X) ## mother(Y,X)),
    father(Z,Y), Z;