docs: protected boolean isValidEventNameForScope(String eventName, Element listenerElement) {
      if (eventName != null && eventName.trim().length() > 0) {
        if ("start".equals(eventName) || "end".equals(eventName)) {
          return true;
        } else {
          addError("Attribute 'event' must be one of {start|end}", listenerElement);
        }
      } else {
        addError("Attribute 'event' is mandatory on listener", listenerElement);
      }
      return false;
    }