anchor.diff:      (assert (zerop (mod n-stripes batch-size)) ()
               "BATCH-SIZE ~S is not a divisor of N-STRIPES ~S." batch-size
               n-stripes)
  -    (flet ((foo (batch-mean batch-stddev)
  +    (flet ((foo (batch-mean batch-variance)
                (with-ones (ones (list batch-size 1))
                  (with-thread-cached-mats
                      ((x-mu* dimensions)
  
positive.code:               (append1 (batch-mean lump) (make-mat (list 1 (size lump)))))))
    (elt (batch-mean lump) subbatch-index))
  
  (defun ensure-batch-stddev (lump subbatch-index)
    (let ((length (length (batch-stddev lump))))
      (if (< subbatch-index length)
          (setf (elt (batch-stddev lump) subbatch-index)
                (adjust! (elt (batch-stddev lump) subbatch-index)
                         (list 1 (size lump)) 0))
          (setf (slot-value lump 'batch-stddev)
                (append1 (batch-stddev lump) (make-mat (list 1 (size lump)))))))
    (elt (batch-stddev lump) subbatch-index))
  
  (defun ensure-population-mean (lump)
    (setf (slot-value lump 'population-mean)
  
negative.code:    ;; A list of means of (SIZE X), one for each subbatch, of which
     ;; there are (/ N-STRIPES BATCH-SIZE).
     (batch-mean :initform nil :accessor batch-mean)
     (batch-variance :initform nil :accessor batch-variance)
     (batch-size
      :initform nil :initarg :batch-size :reader batch-size
      :documentation "Normally all stripes participate in the batch.
  