/**
  * 
  * @return __WebFlowManager
  */
 public static function &getInstance()
 {
     if (self::$_instance == null) {
         self::$_instance = new __WebFlowManager();
     }
     return self::$_instance;
 }
 public static function createFlowExecution($flow_id)
 {
     $return_value = null;
     $flow_definition = __WebFlowManager::getInstance()->getFlowDefinition($flow_id);
     if ($flow_definition != null) {
         $return_value = new __FlowExecution($flow_definition);
     } else {
         throw __ExceptionFactory::getInstance()->createException('Unknown flow for id ' . $flow_id);
     }
     return $return_value;
 }