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

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

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

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

sibling(A, B) :- 
  parent(P, A),
  parent(P, B).

sibling(A, B) :- parent(P, A), parent(P, B), A \== B.

full_sibling(A, B) :-
  father(F, A),
  father(F, B),
  mother(M, A),
  mother(M, B),
  A \== B.
  
orphan(C) :- \+ parent(_, C).

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


% all_equal(A, L) all elements of the list L are A
all_equal(_, []).
all_equal(A, [A|L]) :-
  all_equal(A, L).

same_length([], []).
same_length([_|L1], [_|L2]) :-
  same_length(L1, L2).

convert_to(_, [], []).
convert_to(A, [_|List], [A|AList]) :-
  convert_to(A, List, AList).

my_reverse([], []).
my_reverse([A|L1], L2) :-
  my_reverse(L1, L1rev),
  append(L1rev, [A], L2).

% all_fathers(CS, FS) true if FS are the respective fathers of CS.
all_fathers([], []).
all_fathers([C|CS], [F|FS]) :-
  father(F, C),
  all_fathers(CS, FS).