Ну вот хоть здесь почитайте.http://www.ray-idaho.ru/blog/2016/03/12/%D0%BE%D0%B1%D1%80%D0%B0%D1%82%D0%BD%D0%B0%D1%8F-%D0%B7%D0%B0%D0%B2%D0%B8%D1%81%D0%B8%D0%BC%D0%BE%D1%81%D1%82%D1%8C-%D0%B2%D0%B2%D0%BF-%D0%BD%D0%B0-%D0%B4%D1%83%D1%88%D1%83-%D0%B8-%D1%80%D0%BE%D0%B6/