public static function addMixin($identifier, $mixins)
    {
        settype($mixins, 'array');

        $objIdentifier = self::getIdentifier($identifier);
        $strIdentifier = (string) $objIdentifier;

        if (!isset(self::$_mixins[$strIdentifier])) {
            self::$_mixins[$strIdentifier] = array();
        }

        self::$_mixins[$strIdentifier] = array_unique(array_merge(self::$_mixins[$strIdentifier], $mixins));

        if (self::$_services->offsetExists($strIdentifier)) {
            $instance = self::$_services->offsetGet($strIdentifier);
            self::_mixin($strIdentifier, $instance);
        }
    }