/**
  * instance method
  *
  * Get singleton instance of self (Ai1ec_Database).
  *
  * @return Ai1ec_Database Initialized instance of self
  */
 public static function instance()
 {
     if (!self::$_instance instanceof Ai1ec_Database) {
         self::$_instance = new Ai1ec_Database();
     }
     return self::$_instance;
 }
 /**
  * instance method
  *
  * Get singleton instance of self (Ai1ec_Database).
  *
  * @return Ai1ec_Database Initialized instance of self
  */
 public static function instance()
 {
     if (!self::$_instance instanceof Ai1ec_Database) {
         global $wpdb;
         self::$_instance = new Ai1ec_Database($wpdb);
     }
     return self::$_instance;
 }