private static void NUnknownChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            LinearEquationVM vm     = (LinearEquationVM)d;
            Nullable <int>   newVal = (Nullable <int>)e.NewValue;

            if (newVal.HasValue)
            {
                vm.RebuildCoefficientMatrix();
            }
        }
 public LinearEquationView()
 {
     InitializeComponent();
     _vm = base.DataContext as LinearEquationVM;
 }