/*  File: family.P
/*  Author: Ned Okie 
/*  Date:   8/25/2001
/*  Purpose: To illustrate basic xsb rules and syntax
/*  To invoke prolog compiler: /software/unix/XSB/emu/xsb -i
/*  To compile this file: [family].
/*                                ^ DON'T FORGET THE PERIOD!
/*  To exit the compiler: halt.
/*                            ^ DON'T FORGET THE PERIOD!
*/

male(bob).
parent(jill, bill).
parent(bob, bill).

father(TheDad, TheChild) :- parent(TheDad, TheChild),
                male(TheDad). 

grandparent(TheGP) :- parent(TheGP, AParent), parent(AParent, _ ).