public static function listKnownEntities() {
		$entities = array();
		foreach( static::$knownEntities as $class => &$state ) {
			if( $state == null ) {
				$state = class_exists($class, true) && is_subclass_of($class, 'Orpheus\EntityDescriptor\PermanentEntity');
			}
			if( $state === true ) {
				$entities[] = $class;
			}
		}
		return $entities;
	}