anchor.diff:          ;; obsolete by the prune, and where the corresponding value is a
           ;; list of all dendrites that have reached that buffer.
           (buffer-dendrites-alist (collect-prune-buffer-dendrites-alist buffer))
  -        ;; The sum of the number of elements of all the buffers that are
  -        ;; rendered obsolete by this prune.
  -        (size 0))
  +        ;; The amount of bits of memory freed by applying this prune.
  +        (bits 0))
       (when (null buffer-dendrites-alist)
         (return-from find-prune nil))
       ;; Ensure that each dendrite has a unique associated writer.
       (loop for (buffer . dendrites) in buffer-dendrites-alist do
  -      (incf size (buffer-size buffer))
  +      (incf bits (buffer-bits buffer))
         (loop for dendrite in dendrites do
           (assert (eq buffer (load-instruction-buffer (cdr (dendrite-cons dendrite)))))
           (block check-one-dendrite
  
positive.code:    #'maybe-prune
     (sort (loop for buffer in potentially-superfluous-buffers
                 when (find-prune buffer) collect it)
           #'> :key #'prune-size)))
  
  (defvar *prune*)
  
  
negative.code: ;;; Each individual prune describes how a copy of one writer kernel can be
  ;;; incorporated into one reader kernel.
  (defstruct prune
    ;; The number of bits of memory that are freed by applying this prune.
    (bits nil :type unsigned-byte :read-only t)
    ;; An alist where each key is a kernel, and where each value is a list of
    ;; entries of the form (dendrite writer store-instruction).
    (buffers '() :type list :read-only t))
  