SICP 問題 4.61

;; 先頭の２つの隣接関係
(rule (?x next-to ?y in (?x ?y . ?u)))

;; リストのcdrの隣接関係
;; (1 2 3 4 5)だとvが1,zが(2 3 4 5).２行目で，zに対してもnext-toをやると読める．
(rule (?x next-to ?y in (?v . ?z))
      (?x next-to ?y in ?z))

;; 質問
(?x next-to ?y in (1 (2 3) 4))
;; =>
(1 next-to (2 3) in (1 (2 3) 4))
(2 3) next-to 4 in (1 (2 3) 4)

;; 質問
(?x next-to 1 in (2 1 3 1))
;; =>
(2 next-to 1 in (2 1 3 1))
(3 next-to 1 in (2 1 3 1))