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