public static function isTuple( $taxonomy_element, $types = null )
	{
		if ( $types == null ) $types = XBRL_Types::getInstance();

		return isset( $taxonomy_element['substitutionGroup'] ) &&
		$types->resolveToSubstitutionGroup( $taxonomy_element['substitutionGroup'], array( XBRL_Constants::$xbrliTuple ) );
	}