code: func posBox(m [][]Pixel, x, y int) {
  	pos := Position.Pixel()
  	// box
  	for dy := 0; dy < 7; dy++ {
  		for dx := 0; dx < 7; dx++ {
  			p := pos
  			if dx == 0 || dx == 6 || dy == 0 || dy == 6 || 2 <= dx && dx <= 4 && 2 <= dy && dy <= 4 {
  				p |= Black
  			}
  			m[y+dy][x+dx] = p
  		}
  	}
  	// white border
  	for dy := -1; dy < 8; dy++ {
  		if 0 <= y+dy && y+dy < len(m) {
  			if x > 0 {
  				m[y+dy][x-1] = pos
  			}
  			if x+7 < len(m) {
  				m[y+dy][x+7] = pos
  			}
  		}
  	}
  	for dx := -1; dx < 8; dx++ {
  		if 0 <= x+dx && x+dx < len(m) {
  			if y > 0 {
  				m[y-1][x+dx] = pos
  			}
  			if y+7 < len(m) {
  				m[y+7][x+dx] = pos
  			}
  		}
  	}
  }
docs: // posBox draws a position (large) box at upper left x, y.