anchor.diff:  (defun make-env ()
     '())
  +
  +(defun env-frame-empty-p (env)
  +  (null env))
   
   (defun env-first-frame (env)
     (car env))
  
positive.code:   (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)
  
negative.code:   (symbolp exp))
  
  (defun tagged-list-p (exp tag)
    (and (consp exp) (eq (car exp) tag)))
  
  (defun quoted-p (exp)
    (tagged-list-p exp 'quote))
  