SICP 問題 4.40

人の階への割り当ての組みは，相異なるという要求の前では5⁵通りある．
要求の後では5!通りになる．
　
ambで生成してすぐにテストすることで効率的な手続き

(define (multiple-dwelling)
  (let ((baker (amb 1 2 3 4 5)))
    (require (not (= baker 5)))
    (let ((cooper (amb 1 2 3 4 5)))
      (require (not (= cooper 1)))
      (distinct? (list baker cooper))
      (let ((fletcher (amb 1 2 3 4 5)))
        (require (not (= fletcher 1)))
        (require (not (= fletcher 5)))
        (require (not (= (abs (- fletcher cooper)) 1)))
        (require (distinct? (list baker cooper fletcher)))
        (let ((miller (amb 1 2 3 4 5)))
          (require (< cooper miller))
          (require (distinct? (list baker cooper fletcher miller)))
          (let ((smith (amb 1 2 3 4 5)))
            (require (not (= (abs (- smith fletcher)) 1)))
            (require (distinct? (list baker cooper fletcher miller smith)))
            (list (list 'baker baker)
                  (list 'cooper cooper)
                  (list 'fletcher fletcher)
                  (list 'miller miller)
                  (lsit 'smith smith))))))))