public function SaveUserTree($CategoryID, $Set) {
      $Categories = $this->GetSubtree($CategoryID);
      foreach ($Categories as $Category) {
         $this->SQL->Replace(
            'UserCategory',
            $Set,
            array('UserID' => Gdn::Session()->UserID, 'CategoryID' => $Category['CategoryID']));
      }
      $Key = 'UserCategory_'.Gdn::Session()->UserID;
      Gdn::Cache()->Remove($Key);
   }