問題4.6

(define (let->combination exp)
  (let ((bindings (cadr exp)))
    (let ((vars (map car bindings))
          (exps (map cadr bindings)))
      (append (list (append (list 'lambda vars) (cddr exp))) exps))))

(define (eval-let exp env)
  (eval (let->combitation exp) env))

(put 'eval 'let eval-let)