code: func fplan(l Level, m Mask, p *Plan) error {
  	// Format pixels.
  	fb := uint32(l^1) << 13 // level: L=01, M=00, Q=11, H=10
  	fb |= uint32(m) << 10   // mask
  	const formatPoly = 0x537
  	rem := fb
  	for i := 14; i >= 10; i-- {
  		if rem&(1<<uint(i)) != 0 {
  			rem ^= formatPoly << uint(i-10)
  		}
  	}
  	fb |= rem
  	invert := uint32(0x5412)
  	siz := len(p.Pixel)
  	for i := uint(0); i < 15; i++ {
  		pix := Format.Pixel() + OffsetPixel(i)
  		if (fb>>i)&1 == 1 {
  			pix |= Black
  		}
  		if (invert>>i)&1 == 1 {
  			pix ^= Invert | Black
  		}
  		// top left
  		switch {
  		case i < 6:
  			p.Pixel[i][8] = pix
  		case i < 8:
  			p.Pixel[i+1][8] = pix
  		case i < 9:
  			p.Pixel[8][7] = pix
  		default:
  			p.Pixel[8][14-i] = pix
  		}
  		// bottom right
  		switch {
  		case i < 8:
  			p.Pixel[8][siz-1-int(i)] = pix
  		default:
  			p.Pixel[siz-1-int(14-i)][8] = pix
  		}
  	}
  	return nil
  }
docs: // fplan adds the format pixels