#lang racket

(require "../schelog.rkt")

;This is a very trivial program.  In Prolog, it would be:
;
;    city(amsterdam).
;    city(brussels).
;    country(holland).
;    country(belgium).

(define %city
  (lambda (x)
    (%or (%= x 'amsterdam)
	 (%= x 'brussels))))

(define %country
  (lambda (x)
    (%or (%= x 'holland)
	 (%= x 'belgium))))

;For a more Prolog-style syntax, you can rewrite the same thing,
;using the `%rel' macro, as the following:

'(define %city
  (%rel ()
    (('amsterdam))
    (('brussels))))

'(define %country
  (%rel ()
    (('holland))
    (('belgium))))

;Typical easy queries:
;
; (%which (x) (%city x)) succeeds twice
; (%which (x) (%country x)) succeeds twice
; (%which () (%city 'amsterdam)) succeeds
; (%which () (%country 'amsterdam)) fails