code: func alignBox(m [][]Pixel, x, y int) {
  	// box
  	align := Alignment.Pixel()
  	for dy := 0; dy < 5; dy++ {
  		for dx := 0; dx < 5; dx++ {
  			p := align
  			if dx == 0 || dx == 4 || dy == 0 || dy == 4 || dx == 2 && dy == 2 {
  				p |= Black
  			}
  			m[y+dy][x+dx] = p
  		}
  	}
  }
docs: // alignBox draw an alignment (small) box at upper left x, y.