private IEnumerable <SessionPareOfWords> MixWords(IEnumerable <IPareOfWords> words)
 {
     words = wordManager.MixWords(words);
     words = wordManager.CangeHalfTranslations(words.Cast <SessionPareOfWords>());
     words = wordManager.MixWords(words);
     return((IEnumerable <SessionPareOfWords>)words);
 }