;;Chapter 3, Test 8, Page 47
(deftest test-stacks ()
  (check
    (let ((lst1 nil) (lst2 nil)) ;2 empty lists
      (push 2 lst1) ;adding a value to the first list
      (setf lst2 lst1)  ;makeing the two lists eql.
      (check
        (equal lst1 lst2))  ;testing the equality
      (push 1 lst1) ;adding another element to the first list
      (check
        (not (equal lst1 lst2)))  ;the lists are no longer equal
        (equal (pop lst1) 1)  ;taking the first element off the list. It returns the value
        (equal lst1 lst2) ;the two lists are now equal
        (equal (pushnew 2 lst1) lst2))))  ;push new does not change the list b/c the list already contains 2.