private void _uclSudoku_MouseDown(object sender, MouseButtonEventArgs e) {
         var cll = e.FindParentContext<CellVM>();
         SelectedCell = cll;
         Keyboard.Focus(_uclSudoku);
      }

      private void _uclSudoku_KeyDown(object sender, KeyEventArgs e) {
         if (_SelectedCell == null) return;
         int i = _AllCells.IndexOf(_SelectedCell);
         switch (e.Key) {
         case Key.Right:
            i += 1;
            break;
         case Key.Left:
            i -= 1;
            break;
         case Key.Up:
            i -= 9;
            break;
         case Key.Down:
            i += 9;
            break;
         default:
            return;
         }
         if (i < 0) i += _AllCells.Length;
         else if (i ≥ _AllCells.Length) i -= _AllCells.Length;
         SelectedCell = _AllCells[i];
         e.Handled = true;
      }