anchor.diff:  (defun copy-node (node)
     (copy-array node))
   
  +(defun node-size (node)
  +  (fill-pointer node))
  +
   (defun node-get (node index)
     (declare (fixnum index))
     (aref node index))
  
positive.code:                                 :length length
                                  :height height))))
  
  (defun index-at-height (index height)
    (declare (fixnum index height))
    (logand (rsh index (* height 5)) 31))
  
negative.code: (defun node-push (node value)
    (let ((new-node (copy-node node)))
      (vector-push value new-node)
      new-node))
  
  (defun node-pop (node)
    (let ((new-node (copy-node node)))
      (vector-pop new-node)
      new-node))
  
  (defun node-grow (node height)
  