anchor.diff:  
        ;; We then apply pruning strategies 1 and 2 to the ABL to
        ;; remove unnecessary branches.
  -     (iter (with last = (prune-downward
  +     (iter (with last = (prune-downward-1
                            node point nearest
                            (sort (mapcar (lambda (child)
                                            (let ((mbr (mbr child *tree*)))
  -                                           (vector child
  -                                                   (mindist point mbr)
  -                                                   (minimax-dist point mbr))))
  +                                           (abl-element child
  +                                                        (mindist point mbr)
  +                                                        (minimax-dist point mbr))))
                                          children) #'<
                                  :key #'abl-mindist)))
   
  
positive.code:                  (< min-minimax (abl-mindist node)))
                 abl)))
  
  
  (defun prune-downward-2 (node point nearest abl)
    "2. an actual distance from P to a given object O
  which is greater than the MINMAXDIST(P,M) for
  
negative.code: ;; Originally, our guess for
  ;; the nearest neighbor distance (call it Nearest) is infinity.
  
  (export '(nearest-neighbor-search
            mindist
            minimax-dist
            abl-element
            abl-mindist
            abl-node
            abl-minimax-dist))
  
  (defstruct (abl-element (:constructor abl-element (node mindist minimax-dist))
                          (:conc-name abl-))
    (node nil :type spatial-tree-node)
    (mindist 0 :type number)
    (minimax-dist 0 :type number))
  
  (defstruct (nearest-element (:constructor nearest-element (object dist))
                              (:conc-name nearest-))
    (object nil)
    (dist 0 :type number))
  
  (defvar *tree*)
  (defun nearest-neighbor-search (point *tree* distance-function)
    "point (list numbers*): a list in the same form as lows and highs.
  