/**
  *@singleton method used to instantiate class object
  *@access public
  *@return EE_Registry instance
  */
 public static function instance()
 {
     // check if class object is instantiated
     if (self::$_instance === NULL or !is_object(self::$_instance) or !self::$_instance instanceof EE_Registry) {
         self::$_instance = new self();
     }
     return self::$_instance;
 }
 /**
  * @singleton method used to instantiate class object
  * @access    public
  * @param  \EE_Dependency_Map $dependency_map
  * @return \EE_Registry instance
  */
 public static function instance(\EE_Dependency_Map $dependency_map = null)
 {
     // check if class object is instantiated
     if (!self::$_instance instanceof EE_Registry) {
         self::$_instance = new EE_Registry($dependency_map);
     }
     return self::$_instance;
 }