father(frank, jane).
father(james, anne).
father(frank, tim).
father(john, frank).
mother(anne, jane).
mother(betty, frank).

parent(P, C) :- father(P, C).
parent(P, C) :- mother(P, C).

parents(F, M, C) :- father(F, C), mother(M, C).

grandparent(G, C) :- parent(G, P), parent(P, C).

sibling(A, B) :- parent(P, A), parent(P, B),
  A \= B.
orphan(C) :- \+ parent(_, C).

ancestor(A, D) :- parent(A, P), ancestor(P, D).
% ancestor(P, C) :- parent(P, C).
ancestor(P, P).

all_equal(_, []).
all_equal(A, [A|Tail]) :- all_equal(A, Tail).

same_length([], []).
same_length([_|A], [_|B]) :- same_length(A, B).

my_reverse([], []).
my_reverse([A|Tail], Rev) :-
  my_reverse(Tail, RevTail),
  append(RevTail, [A], Rev).

all_fathers([], []).
all_fathers([F|FS], [C|CS]) :-
  father(F, C),
  all_fathers(FS, CS).