@Override
  public DomExtension addExtender(final DomExtender extender) {
    if (myUserMap == null || !myUserMap.containsKey(DOM_EXTENDER_KEY)) {
      putUserData(DOM_EXTENDER_KEY, new SmartList<>());
    }
    //noinspection unchecked
    ((List<DomExtender<?>>)myUserMap.get(DOM_EXTENDER_KEY)).add(extender);
    return this;
  }
@Override
  public DomExtension addExtender(final DomExtender extender)