/**
  * Adds an entry to {@link Notification#getArguments()}. Existing entries will be overwritten.
  *
  * @param argumentName The name of the argument.
  * @param argumentValue The value of the argument.
  * @return The builder.
  */
 public B argument(String argumentName, Object argumentValue) {
   notification.getArguments().put(argumentName, argumentValue);
   return self();
 }