code: public static void notFound(Router router, String regExPath, Class<? extends NotFoundResponseWriter> notFound) {
  
  		Assert.notNull(router, "Missing router!");
  		Assert.notNull(notFound, "Missing not found handler!");
  
  		addLastHandler(router, regExPath, getNotFoundHandler(notFound));
  	}
docs: Handles not found route in case request regExPath mathes given regExPath prefix
  
  @param router   to add route to
  @param regExPath     prefix
  @param notFound hander