anchor.diff:    (rplaca frame (cons var (frame-vars frame)))
     (rplacd frame (cons val (frame-vals frame)))
     'ok)
  -
  -(defun frame-define! (frame var val)
  -  (frame-set! frame var val t))
  -
  -(defun frame-set! (frame var val &optional (define nil))
  -  (labels ((iter (vars vals)
  -	     (if (null vars)
  -		 (if define
  -		     (frame-append-var-val! frame var val)
  -		     (error "undefined variable: ~a" var))
  -		 (if (eq var (car vars))
  -		     (rplaca vals val)
  -		     (iter (cdr vars) (cdr vals))))))
  -    (iter (frame-vars frame) (frame-vals frame))
  -    'ok))
   
   (defun frame-get (frame var)
     (labels ((iter (vars vals)
  
positive.code: (defun make-env ()
    '())
  
  (defun env-first-frame (env)
    (car env))
  
negative.code: (defun eval-definition (exp env)
    (let ((var (definition-var exp))
  	(val (definition-val exp)))
      (define-variable! (env-first-frame env)
  	var
        (t-eval val env))
      'ok))
  
  (defun eval-assignment (exp env)
    (format t "eval-assignment: exp=~a~%" exp)
    (format t "eval-assignment: env=~a~%" env)
    (let ((var (assignment-var exp))
  	(val (eval (assignment-val exp))))
      (set-variable-value! env var val)
      'ok))
  
  (defun define-variable! (frame var val)
    (labels ((iter (vars vals)
  	     (if (null vars)
  		 (frame-append-var-val! frame var val)
  		 (if (eq var (car vars))
  		     (rplaca vals val)
  		     (iter (cdr vars) (cdr vals))))))
      (iter (frame-vars frame) (frame-vals frame))))
  
  (defun set-variable-value! (env var val)
    (labels ((iter-frame (env)
  	     (labels ((iter (vars vals)
  			(if (null vars)
  			    (iter-frame (env-rest env))
  			    (if (eq var (car vars))
  				(rplaca vals val)
  				(iter (cdr vars) (cdr vals))))))
  	       (if (env-frame-empty-p env)
  		 (error "undefined variable: ~a" var)
  		 (let ((frame (env-first-frame env)))
  		   (iter (frame-vars frame) (frame-vals frame)))))))
      (iter-frame env)))
  
  (defun lambda-p (exp)
    (tagged-list-p exp 'lambda))
  