public void ClearTest()
    {
      ITvCardContext ctx = new TvCardContext();

      for (int i = 0; i < 10; i++)
      {
        IUser u1 = new User("u" + i, false, 1);
        ctx.Add(u1);
      }
      ctx.Clear();
      Assert.AreEqual(10, ((TvCardContext)ctx).UsersOld.Count, "oldusers count wrong");
      Assert.AreEqual(0, ctx.Users.Length, "users count wrong");
    }