(define (add-binding-to-frame! var val frame)
  (set! frame (cons (cons var val) frame)))

(define (extend-environment bindings base-env)
  (cons bindings base-env))

(define (lookup-variable-value var env)
  (define (env-loop env)
    (define (scan bindings)
      (cond ((null? bindings)
             (env-loop (enclosing-environment env)))
            ((eq? var (caar bindings))
             (cdar bindings))
            (else 
              (scan (cdr bindings)))))
    (if (eq? env the-empty-environment)
        (error "Unbound variable" var)
        (let ((frame (first-frame env)))
          (scan frame))))
  (env-loop env))

(define (set-variable-value! var val env)
  (define (env-loop env)
    (define (scan bindings)
      (cond ((null? bindings)
             (env-loop (enclosing-environment env)))
            ((eq? var (caar bindings))
             (set-cdr! (car bindings) val))
            (else
              (scan (cdr bindings)))))
    (if (eq? env the-empty-environment)
        (error "Unbound variable" var)
        (let ((frame (first-frame env)))
          (scan fram))))
  (env-loop env))

(define (define-variable! var val env)
  (let ((frame (first-frame env)))
    (define (scan bindings)
      (cond ((null? bindings)
             (add-binding-to-frame! var val frame))
            ((eq? var (caar bindings))
             (set-car! (car bindings) val))
            (else (scan (cdr bindings)))))
    (scan frame)))