code: protected int insertPlaceholderAsLastChild(Element placeholder) {
  
          m_children.getElement().appendChild(placeholder);
          getTree().setPlaceholderPath(getPath());
          return getChildCount();
      }
docs: Inserts the placeholder element as last child of the children list.
  Setting it's path as the current placeholder path and returning the new index.<p>
  
  @param placeholder the placeholder element
  
  @return the new index