anchor.diff:  					       (mapcar #'(lambda (y)
   							   (if (eq sym2 (get-sym y))
   							       (cons sym2
  -								     (cons item (get-val y)))
  +								     (cons item nil))
   							       y))
   						       (get-val x)))
   					 x))
  
positive.code: (defun my-get (sym1 sym2)
    (labels ((lookup (sym lst)
  	     (cond ((null lst) nil)
  		   ((eq sym (get-sym (car lst)))
  		    (get-val (car lst)))
  		   (t (lookup sym (cdr lst))))))
      (lookup sym2 (lookup sym1 *datum*))))
  
  
  (defun install-rectangular-package ()
  
negative.code: 
  ;;;;基于类型的分派
  (defun attach-tag (type-tag contents)
    (cons type-tag contents))
  (defun type-tag (datum)
    (if (consp datum)
        (car datum)
  