SICP 問題 4.63

SはGの孫であるという規則の形式化

(rule (?son son-of ?dad)
      (or (son ?dad ?son)
          (and (wife ?dad ?mam)
               (son ?mam ?son))))

(rule (?grandson grandson-of ?granddad)
      (and (?parent son-of ?grandson)
           (?grandson son-of ?parent)))