/* family database */

is_parent_of(pam, bob).
is_parent_of(tom, bob).
is_parent_of(tom, liz).
is_parent_of(bob, ann).
is_parent_of(bob, pat).
is_parent_of(pat, jim).

female(pam).
female(liz).
female(pat).
female(ann).
male(tom).
male(bob).
male(jim).

is_mother_of(X,Y) :-
    is_parent_of(X,Y),
    female(X).
is_sister_of(X,Y) :-
    is_parent_of(Z,X),
    is_parent_of(Z,Y), 
    female(X), dif(X,Y).

has_a_child(X) :- is_parent_of(X,_).

is_ancestor_of(X,Y) :-
    is_parent_of(X,Y).

is_ancestor_of(X,Y) :- 
    is_parent_of(X,Z),
    is_ancestor_of(Z,Y).