anchor.diff:      (is (equal 'a (car loc)))
       (is (equal '2 (car new-loc)))
       (is (equal '(*) (lefts new-loc)))
  -    (is (equal '() (rights new-loc)))
  +    (is (null (rights new-loc)))
       (is (equal '(/ (+ (* 2) (- b 4))) (root-node new-loc)))))
   
   (test remove-node-at-leftmost-node
  
positive.code:   (is (null (go-left (-> *tree*)))))
  
  (test go-left-at-nil
  
    (is (null (go-left nil))))
  
  (test go-left-at-end
  
negative.code:          (new-loc (remove-node loc)))
      (is (equal '* (car loc)))
      (is (equal 'a (car new-loc)))
      (is (null (lefts new-loc)))
      (is (equal '(2) (rights new-loc)))
      (is (equal '(/ (+ (a 2) (- b 4))) (root-node new-loc)))))
  
  