private void self_Loaded(object sender, RoutedEventArgs e)
        {
            tb.IsMouseCapturedChanged += new EventHandler(ImplementQuntity_IsMouseCapturedChanged);
        }
        private void ImplementQuntity_IsMouseCapturedChanged(object sender, DependencyPropertyChangedEventArgs e)
        {
            if (ImplementQuntity.Text.Length > 0)
            {
                int a = ImplementQuntity.Text.IndexOf("+", ImplementQuntity.SelectionStart);
                if (a > 0)
                {
                    int b = ImplementQuntity.Text.LastIndexOf("+", a - 1);
                    ImplementQuntity.Select(b + 1, a - b - 1);
                }
                else if (a == -1)
                {
                    int b = ImplementQuntity.Text.LastIndexOf("+");
                    ImplementQuntity.Select(b + 1, ImplementQuntity.Text.Length);
                }
                double s = Convert.ToDouble(ImplementQuntity.SelectedText);
                if (FindTreeViewItem(element, s) != null)
                    FindTreeViewItem(element, s).IsSelected = true;
            }
        }