anchor.diff:  ;;;      (* WEIGHT-DECAY WEIGHTS))
   ;;;
   ;;; plus momentum, weight-penalty.
  -(defun update-all-weights (trainer)
  +(defun update-all-weights/nesterov (trainer)
     (map nil #'funcall (before-update-hook trainer))
     (let ((accumulator (accumulator trainer))
           (weight-deltas (weight-deltas trainer))
  
positive.code: (defmethod maybe-update-weights ((trainer batch-gd-trainer) n-new-inputs)
    (when (<= (batch-size trainer)
              (incf (n-inputs-in-batch trainer) n-new-inputs))
      (update-all-weights trainer))
    (incf (n-inputs trainer) n-new-inputs))
  
  ;;; delta_w' += m * delta_w + df/dw
  
negative.code: in the batch or the number of uses the weight in question has seen.")
     (momentum
      :initform #.(flt 0) :initarg :momentum :accessor momentum)
     (momentum-type
      :initform :normal :initarg :momentum-type :accessor momentum-type
      :type '(member :normal :nesterov))
     (weight-decay
      :initform #.(flt 0) :initarg :weight-decay :accessor weight-decay
      :documentation "WEIGHT-DECAY * WEIGHT is added to the gradient to
  