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