anchor.diff:      ((self-evaluating-p exp) exp)
       ((variable-p exp) (env-get env exp))
       ((quoted-p exp) (object-of-quoted exp))
  +    ((definition-p exp) (eval-definition exp env))
       (t 'not-implemented)))
   
   (defun self-evaluating-p (exp)
  
positive.code: (defun object-of-quoted (exp)
    (cadr exp))
  
  (defun t-print (exp)
    (print exp))
  
  (defun t-apply ())
  
  (defun repl ()
    (let ((env (make-env)))
     (loop
       (fresh-line)
       (format t "LISP> ")
       (t-print (t-eval (read) env)))))
  
  
negative.code: 
  (defun env-get (env var)
    (if (null env)
        (error "undefined variable: ~a" var)
        (multiple-value-bind (val exist) (frame-get (env-first-frame env) var)
  	(if (null exist)
  	    (env-get (env-rest env) var)
  