/* CS 1621 Fall 2005
   Simple example of a Prolog program.  Note the "propositional" nature
   of the language.  The assertion succeeds or fails based on the
   data (atoms) stored.
*/

cousin(X,Y) :- X \== Y, parent(P1,X), parent(P2,Y), sibling(P1,P2).

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

sibling(X,Y) :- X \== Y, parent(P,X), parent(P,Y).

mother(mary, irving).
mother(mary, martha).
mother(martha, fred).
mother(martha, jethro).

father(herb, irving).
father(herb, ingmar).
father(irving, mitch).
father(irving, amy).
father(ingmar, slim).