% genealogy.dl -- Classic Datalog genealogy example
%
% Demonstrates facts, recursive rules, and queries using
% a family tree database.

% --- Facts: the family tree ---

parent(tom, bob).
parent(tom, liz).
parent(bob, ann).
parent(bob, pat).
parent(pat, jim).
parent(liz, mia).

% Gender facts
male(tom).
male(bob).
male(pat).
male(jim).
female(liz).
female(ann).
female(mia).

% --- Rules ---

% Direct parent-child relationships
father(X, Y) :- parent(X, Y), male(X).
mother(X, Y) :- parent(X, Y), female(X).

% Grandparent
grandparent(X, Z) :- parent(X, Y), parent(Y, Z).

% Siblings share a parent
sibling(X, Y) :- parent(Z, X), parent(Z, Y), X \= Y.

% Ancestor (recursive)
ancestor(X, Y) :- parent(X, Y).
ancestor(X, Y) :- parent(X, Z), ancestor(Z, Y).

% Descendant (symmetric to ancestor)
descendant(X, Y) :- ancestor(Y, X).

% --- Queries ---

?- grandparent(tom, X).
?- ancestor(tom, jim).
?- sibling(ann, X).
?- descendant(jim, X).