anchor.diff:             (type (simple-array (unsigned-byte 32) (*)) to)
              (type (simple-array bit (*)) mask)
              (type (unsigned-byte 32) colour))
  -  (dotimes (i ncols)
  +  (loop for i fixnum below ncols do
       (when (not (eql (aref mask mask-offset) 0))
         (funcall blender colour to to-offset))
       (incf to-offset)
       (incf mask-offset)))
   
  -(declaim (inline %bitset-mask-8-line))
   (defun %bitset-mask-8-line (blender to to-offset ncols mask mask-offset colour)
     (declare (optimize speed (safety 0) (debug 0))
              (type function blender)
  
positive.code:            (type (simple-array (unsigned-byte 32) (*)) to)
             (type (simple-array (unsigned-byte 8) (*)) mask)
             (type (unsigned-byte 32) colour))
    (dotimes (i ncols)
      (let ((mask-byte (aref mask mask-offset)))
        (cond ((eql mask-byte 0)
               ;; Do nothing.
  
negative.code:            (type fixnum to-offset))
    (setf (aref to to-offset) source))
  
  (defun %%xor-one-argb8888-argb8888 (source to to-offset)
    (declare (optimize speed (safety 0) (debug 0))
             (type (unsigned-byte 32) source)
  