% Here we introduce a bunch of predicates representing many family
% relationships. The comma indicates logical and, so mother is implied by
% being a parent AND being female.
mother(X, Y) :-
	parent(X, Y),
	female(X).

father(X, Y) :-
	parent(X, Y),
	male(X).

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

daughter(X, Y) :-
	child(X, Y),
	female(X).

son(X, Y) :-
	child(X, Y),
	male(X).

% Here we use a not-equal comparison, as well as a a throwawy variable, Z,
% which represents the person who is parent to both X and Y. Prolog will
% find a correct binding for Z if one is possible.
% If there isn't a correct way to bind Z, the program will fail
sibling(X, Y) :-
	X \== Y,
	child(X, Z),
	child(Y, Z).

sister(X, Y) :-
	sibling(X, Y),
	female(X).

brother(X, Y) :-
	sibling(X, Y),
	male(X).

uncle(X, Y) :-
	parent(Z, Y),
	brother(X, Z).

aunt(X, Y) :-
	parent(Z, Y),
	sister(X, Z).

cousin(X, Y) :-
	parent(Z, Y),
	parent(W, X),
	sibling(Z, W).

% Here we see a logical or (symbolized by semicolon)
relative(X, Y) :-
	parent(X, Y);
	child(X, Y);
	sibling(X, Y).

% You see we've now declared the same relative(X, Y) predicate twice. The
% two definitions are logically or'd together in the Prolog system.
relative(X, Y) :-
	child(X, Z),
	relative(Y, Z).

relative(X, Y) :-
	child(Y, Z),
	relative(X, Z).


female(mary).
female(jane).
female(joan).
female(beth).
female(sally).
female(francis).

male(mark).
male(mel).
male(john).
male(bob).
male(salvador).

parent(mary, jane).
parent(mary, john).
parent(mary, joan).
parent(mark, jane).
parent(mark, john).
parent(mel, joan).
parent(jane, beth).
parent(jane, bob).
parent(john, salvador).
parent(john, sally).
parent(joan, francis).