wpf – 如何在UIElement.Margin上为绑定设置FallbackValue？

<Border BorderBrush="#cccccc" BorderThickness="1" Margin="{Binding PushMargin,FallbackValue=0 60 0 0}"></Border>

<Border BorderBrush="#cccccc" BorderThickness="1" Margin="{Binding PushMargin,FallbackValue=0 60 0 0}"></Border>

设置方法，请参考黄色背景位置。用空格隔开。