code: func Register(bcode barcode.Barcode) string {
  	barcodes.Lock()
  	if len(barcodes.cache) == 0 {
  		barcodes.cache = make(map[string]barcode.Barcode)
  	}
  
  	key := barcodeKey(bcode)
  	barcodes.cache[key] = bcode
  	barcodes.Unlock()
  
  	return key
  }
docs: // Register registers a barcode but does not put it on the page. Use Barcode()
  // with the same code to put the barcode on the PDF page.