#lang syndicate/hll ;; -*- racket -*-

(actor (forever (assert `(parent john douglas))))
(actor (forever (assert `(parent bob john))))
(actor (forever (assert `(parent ebbon bob))))

(actor (forever (on (asserted `(parent ,$A ,$C))
                    (until (retracted `(parent ,A ,C))
                      (assert `(ancestor ,A ,C))
                      (on (asserted `(ancestor ,C ,$B))
                          (until (retracted `(ancestor ,C ,B))
                            (assert `(ancestor ,A ,B))))))))