code: public static void set(Message message, ExplicitMessageEncryptionProtocol protocol) {
          if (!hasProtocol(message, protocol.namespace)) {
              message.addExtension(new ExplicitMessageEncryptionElement(protocol));
          }
      }
docs: Add an EME element containing the specified {@code protocol} namespace to the message.
  In case there is already an element with that protocol, we do nothing.
  
  @param message message
  @param protocol encryption protocol