code: func (q *QRCode) ToString(inverseColor bool) string {
  	bits := q.Bitmap()
  	var buf bytes.Buffer
  	for y := range bits {
  		for x := range bits[y] {
  			if bits[y][x] != inverseColor {
  				buf.WriteString("  ")
  			} else {
  				buf.WriteString("██")
  			}
  		}
  		buf.WriteString("\n")
  	}
  	return buf.String()
  }
docs: // ToString produces a multi-line string that forms a QR-code image.