anchor.diff:          on (rest (clouds-up-to-layers dbm))
           while layer
           do (swap-nodes layer-below)
  +        ;; This is an alternative implementation that doubles the
  +        ;; biases from below.
  +        #+nil
  +        (let* ((layer*
  +                (set-difference layer (visible-and-conditioning-chunks dbm)))
  +               (layer-above*
  +                (set-difference layer-above
  +                                (visible-and-conditioning-chunks dbm))))
  +          (hijack-means-to-activation layer* clouds-up-to-layer
  +                                      dbm)
  +          ;; Double activations of chunks in LAYER that have non-bias
  +          ;; connections to LAYER-ABOVE.
  +          (dolist (chunk layer*)
  +            (when (connects-to-p chunk layer-above* clouds-up-to-layer-above)
  +              (matlisp:scal! #.(flt 2) (nodes chunk))))
  +          (map nil #'set-chunk-mean layer*)
  +          (swap-nodes layer-below))
           (let* ((layer*
                   (set-difference layer (visible-and-conditioning-chunks dbm)))
                  (layer-above*
  
positive.code:    (sparsity-target
      :type flt
      :initarg :sparsity :initarg :target :initarg :sparsity-target
      :reader sparsity-target)
     (cost :type flt :initarg :cost :reader cost)
     (damping :type flt :initarg :damping :reader damping)
     (products :type flt-vector :initarg :products :reader products)
  
negative.code:           (dolist (chunk layer*)
              (when (connects-to-p chunk layer-above* clouds-up-to-layer-above)
                (matlisp:scal! #.(flt 2) (nodes chunk))))
            ;; Activations from conditioning chunks are not doubled.
            ;; Normally, there are biases both in the layer below and
            ;; above, while non-bias conditioning chunks are visible.
            (hijack-means-to-activation layer* conditioning-clouds dbm
                                        :addp t)
            (hijack-means-to-activation layer* conditioning-clouds-above dbm
                                        :addp t)
            (map nil #'set-chunk-mean layer*)
            (swap-nodes layer-below))))
  
  (defun down-dbm (dbm)
  