SICP問題4.13

束縛を除去するunbind!式の定義。
unbind!の実装は最初のフレームからだけ結合を除去する仕様とした。
これはdefineが最初のフレームに束縛を追加するのと対応づけるためである。

(define (unbind? exp)
  (tagged-list? exp 'unbind!))
(define (eval-unbind! exp env)
  (unbind-variable! (definition-variable exp) env))
(define (unbind-variable! var env)
  (let ((frame (first-frame env)))
    (let ((result (scan var
                        (frame-variables frame)
                        (frame-values frame)
                        (lambda (vars vals)
                          (set-car! vars (cdr vars))
                          (set-car! vals (cdr vals))
                          #t))))
      (if (not result)
          (error "Unbound variable --UNBIND!" var)))))

evalにも組み込む

(define (eval exp env)
  (cond ((self-evaluating? exp) exp)
        ((variable? exp) (lookup-variable-value exp))
        ((quoted? exp) (text-of-quotation exp))
        ((assignment? exp) (eval-assignment exp env))
        ((definition? exp) (eval-definition exp env))
        ((unbind? exp) (eval-unbind! exp env)) ; 追加
        ((if? exp) (eval-if exp env))
        ((lambda? exp)
         (make-procedure (lambda-parameters exp)
                         (lambda-body exp)
                         env))
        ((let? exp) (eval (let->combination exp) env))
        ((let*? exp) (eval (let*->nested-lets exp) env))
        ((while? exp) (eval (while->named-let exp) env))
        ((begin? exp)
         (eval-sequence (begin-actions exp) env))
        ((cond? exp) (eval (cond->if exp) env))
        ((and? exp) (eval-and exp env))
        ((or? exp) (eval-or exp env))
        ((application? exp)
         (apply (eval (operator exp) env)
                (list-of-values (operands exp) env)))
        (else
         (error "Unknown expression type -- EVAL" exp))))