anchor.diff:      (is (equal '2 (car loc)))
       (is (equal 'a (car new-loc)))
       (is (equal '(*) (lefts new-loc)))
  -    (is (equal '() (rights new-loc)))
  +    (is (null (rights new-loc)))
       (is (equal '(/ (+ (* a) (- b 4))) (root-node new-loc)))))
   
   (test remove-node-at-nil
  
positive.code: 
  (defun -> (tree &rest path)
    "Navigates the given path on tree, where each element in path is a keyword
  that represents the desired direction."
    (labels ((nav (loc rest)
                  (if rest
                      (nav (case (car rest)
                             (:down (go-down loc))
                             (:up (go-up loc))
                             (:left (go-left loc))
                             (:right (go-right loc)))
                           (cdr rest))
                    loc)))
      (nav (zipper tree) path)))
  
negative.code:     (is (equal 'a (car loc)))
      (is (equal '2 (car new-loc)))
      (is (equal '(*) (lefts new-loc)))
      (is (null (rights new-loc)))
      (is (equal '(/ (+ (* 2) (- b 4))) (root-node new-loc)))))
  
  (test remove-node-at-leftmost-node
  