;;;; -*- Mode: Lisp; Syntax: Common-Lisp -*-
;;;; Code from Paradigms of AI Programming
;;;; Copyright (c) 1991 Peter Norvig

(<- (likes Kim Robin))
(<- (likes Sandy Lee))
(<- (likes Sandy Kim))
(<- (likes Robin cats))
(<- (likes Sandy ?x) (likes ?x cats))
(<- (likes Kim ?x) (likes ?x Lee) (likes ?x Kim))
(<- (likes ?x ?x))

(?- (likes Sandy ?who))
;;;; ?WHO = LEE;
;;;; ?WHO = KIM;
;;;; ?WHO = ROBIN;
;;;; ?WHO = SANDY;
;;;; ?WHO = CATS;
;;;; ?WHO = SANDY;

(?- (likes ?who Sandy))
;;;; ?WHO = SANDY;
;;;; ?WHO = KIM;
;;;; ?WHO = SANDY;

(?- (likes Robin Lee))
;;;; No.

(?- (likes ?x ?y) (likes ?y ?x))
;;;; ?Y = KIM
;;;; ?X = SANDY;
;;;; ?Y = SANDY
;;;; ?X = SANDY;
;;;; ?Y = SANDY
;;;; ?X = SANDY;
;;;; ?Y = SANDY
;;;; ?X = KIM;
;;;; ?Y = SANDY
;;;; ?X = SANDY;
;;;; ?Y = ?X3251
;;;; ?X = ?X3251;