@NotNull
  @Override
  public Collection<VirtualFile> getVirtualFiles() {
    ArrayList<VirtualFile> result = new ArrayList<>();
    for (int i = 0; i < getChildCount(); i++) {
      result.addAll(((AbstractTreeNode)getChildAt(i)).getVirtualFiles());
    }
    return result;
  }
@NotNull
  @Override
  public Collection<VirtualFile> getVirtualFiles()