anchor.diff:             (type (simple-array (unsigned-byte 32) (*)) to)
              (type (simple-array (unsigned-byte 8) (*)) mask)
              (type (unsigned-byte 32) colour))
  -  (dotimes (i ncols)
  +  (loop for i fixnum below ncols do
       (let ((mask-byte (aref mask mask-offset)))
         (cond ((eql mask-byte 0)
                ;; Do nothing.
  
positive.code: 
  ;; Line traversal functions.
  
  (declaim (inline %bitblt-line))
  (defun %bitblt-line (blender to to-offset ncols from from-offset)
    (declare (optimize speed (safety 0) (debug 0))
             (type function blender)
             (type fixnum to-offset ncols from-offset)
             (type (simple-array (unsigned-byte 32) (*)) to from))
    (dotimes (i ncols)
      (funcall blender (aref from from-offset) to to-offset)
      (incf to-offset)
      (incf from-offset)))
  
  (declaim (inline %bitset-line))
  (defun %bitset-line (blender to to-offset ncols colour)
    (declare (optimize speed (safety 0) (debug 0))
             (type function blender)
             (type fixnum to-offset ncols)
             (type (simple-array (unsigned-byte 32) (*)) to)
             (type (unsigned-byte 32) colour))
    (dotimes (i ncols)
      (funcall blender colour to to-offset)
      (incf to-offset)))
  
  (declaim (inline %bitset-mask-1-line))
  (defun %bitset-mask-1-line (blender to to-offset ncols mask mask-offset colour)
    (declare (optimize speed (safety 0) (debug 0))
             (type function blender)
  
negative.code:            (type (simple-array (unsigned-byte 32) (*)) to)
             (type (simple-array bit (*)) mask)
             (type (unsigned-byte 32) colour))
    (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)))
  
  (defun %bitset-mask-8-line (blender to to-offset ncols mask mask-offset colour)
    (declare (optimize speed (safety 0) (debug 0))
             (type function blender)
  