code: func mplan(m Mask, p *Plan) error {
  	p.Mask = m
  	for y, row := range p.Pixel {
  		for x, pix := range row {
  			if r := pix.Role(); (r == Data || r == Check || r == Extra) && p.Mask.Invert(y, x) {
  				row[x] ^= Black | Invert
  			}
  		}
  	}
  	return nil
  }
docs: // mplan edits a version+level-only Plan to add the mask.