anchor.diff:      (t (- (diet-height (s-diet-node-right a-tree))
             (diet-height (s-diet-node-left a-tree))))))
   
  -
   (defun diet-max-interval (a-tree)
     (cond
       ((null a-tree) 0)
       ((and (null (s-diet-node-left a-tree)) (null (s-diet-node-right a-tree)))
  -         (1+ (- (s-diet-node-top a-tree) (s-diet-node-bottom a-tree))))
  +     (1+ (- (s-diet-node-top a-tree) (s-diet-node-bottom a-tree))))
       ((null (s-diet-node-left a-tree))
        (max (1+ (- (s-diet-node-top a-tree) (s-diet-node-bottom a-tree)))
             (diet-max-interval (s-diet-node-right a-tree))))
  
positive.code:           (diet-max-interval (s-diet-node-right a-tree))))))
  
  
  
negative.code:          ;; advance to a left subtree
           ((< a-val bottom)
            (make-s-diet-node :bottom bottom :top top :left (diet-remove a-val left) :right right))
           ;; Higher than a top boundary
           ;; advance to a right subtree
           ((> a-val top)
            (make-s-diet-node :bottom bottom :top top :left left :right (diet-remove a-val right)))
  