SICP 問題 4.69

((great great grandson) adam Irad)のような質問ができるようにする．

(rule (greatson-end ?x)
      (append-to-form ?u (grandson) ?x))

(rule ((grandson) ?x)
      (grandson ?x))
(rule ((great . ?rel) ?x ?y)
      (and (greatson-end ?rel)
           (son-of ?x ?z)
           (?rel ?z ?y)))