#lang datalog/sexp
; Equality test
(! (:- (ancestor A B)
       (parent A B)))
(! (:- (ancestor A B)
       (parent A C)
       (= D C) ; Unification required
       (ancestor D B)))
(! (parent john douglas))
(! (parent bob john))
(! (parent ebbon bob))
(? (ancestor A B))