@NotNull
    @Override
    Collection<AbstractTreeNode<?>> createChildren(@NotNull Collection<? extends AbstractTreeNode<?>> old) {
      Group group = this.group;
      if (group == null) return Collections.emptyList();
      RootNode node = group.getSingleRoot();
      if (node == null) {
        return group.createChildren(this, old);
      }
      node.setParent(this);
      return node.getChildren();
    }
@NotNull
    @Override
    Collection<AbstractTreeNode<?>> createChildren(@NotNull Collection<? extends AbstractTreeNode<?>> old)