anchor.diff:  (defmacro do-stripes ((chunk &optional (stripe (gensym))) &body body)
     (with-gensyms (%chunk)
       `(let ((,%chunk ,chunk))
  -       (assert (locally (declare (optimize (speed 1)))
  -                 (or (not (indices-present ,%chunk))
  -                     (= 1 (chunk-n-stripes ,%chunk)))))
  +       (check-stripes ,%chunk)
          (dotimes (,stripe (chunk-n-stripes ,%chunk))
            (let ((*current-stripe* ,stripe))
              ,@body)))))
  
positive.code:                                accumulator
                                 accumulator2)))
            (if (and (zerop start)
                     (null (indices-present (visible-chunk cloud))))
                (matlisp:gemm! (flt 1) v b (flt 0) x :tt)
                (progn
                  (assert (= 1 (n-stripes visible-chunk)))
                  (let ((b* (storage b)))
                    (declare (optimize (speed 3) #.*no-array-bounds-check*))
                    (matlisp:fill-matrix x (flt 0))
                    (do-stripes (visible-chunk)
                      (do-chunk (i visible-chunk)
                        (let ((v*i (aref v* i)))
                          (unless (zerop v*i)
                            (loop for j of-type index upfrom 0 below c
                                  for bi of-type index
                                  upfrom (the! index (* i c)) do
                                  (incf (aref shared* j)
                                        (* v*i (aref b* bi)))))))))))
            (matlisp:gemm! (flt (if addp 1 -1)) h x
                           (flt 1) (reshape2 accumulator
                                             (matlisp:nrows a)
  
negative.code: 
  (defmethod initialize-instance :after ((chunk constant-chunk)
                                         &key &allow-other-keys)
    (fill-chunk chunk (default-value chunk) :allp t))
  
  (defclass sigmoid-chunk (chunk) ()
    (:documentation "Nodes in a sigmoid chunk have two possible samples:
  