code: protected void addTransportContribution(
  			LayerContributionType type,
  			Object arg) throws IOException {
  
  		String transportContrib = transport.getLayerContribution(
  				request,
  				type,
  				arg
  				);
  		if (transportContrib != null) {
  			writer.append(transportContrib);
  		}
  	}
docs: Appends the layer contribution specified by {@code type}
  (contributed by the transport) to the string buffer.
  
  @param type
  The layer contribution type
  @param arg
  The argument value (see
  {@link IHttpTransport#contributeLoaderExtensionJavaScript(String)}
  @throws IOException