если есть время,почитайте
http://books.google.ru/books?id=sVAWNNHuGcYC&pg=PA61&lpg=PA61&dq=%D0%B1%D1%80%D0%B0%D1%82%D1%8C%D1%8F+%D0%BA%D0%B0%D1%80%D0%B0%D0%BC%D0%B0%D0%B7%D0%BE%D0%B2%D1%8B+%D0%B8+%D0%BF%D1%81%D0%B8%D1%85%D0%BE%D0%B0%D0%BD%D0%B0%D0%BB%D0%B8%D0%B7&source=bl&ots=v9bvrnW7DU&sig=71_VW7fPEAmPdFl2QmJTAN0k78M&hl=ru&sa=X&ei=GytcUv7mM-aG4gSf9IHIDA&ved=0CEgQ6AEwBQ#v=onepage&q=%D0%B1%D1%80%D0%B0%D1%82%D1%8C%D1%8F%20%D0%BA%D0%B0%D1%80%D0%B0%D0%BC%D0%B0%D0%B7%D0%BE%D0%B2%D1%8B%20%D0%B8%20%D0%BF%D1%81%D0%B8%D1%85%D0%BE%D0%B0%D0%BD%D0%B0%D0%BB%D0%B8%D0%B7&f=false