public static function skeleton_properties($type)
	{
		$class = get_called_class();

		if (isset(static::$_skeleton_cached[$class][$type]))
		{
			return static::$_skeleton_cached[$class][$type];
		}

		$var = '_' . $type . '_properties';
		$properties = [];

		if (property_exists($class, $var))
		{
			$properties = static::$$var;
		}

		return static::$_skeleton_cached[$class][$type] = static::compile_properties($properties);
	}