/**
  * Generates a unique id for a builder node such as a
  * row, column or module. 
  *
  * @since 1.0
  * @return string
  */
 public static function generate_node_id()
 {
     $node_id = uniqid();
     if ($node_id == self::$last_generated_node_id) {
         return self::generate_node_id();
     }
     self::$last_generated_node_id = $node_id;
     return $node_id;
 }