WPF:如何获得StackPanel中的剩余宽度?

给出以下代码:

<Window x:Class="Window3"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window3" Height="300" Width="300">
    <StackPanel Background="Yellow" Orientation="Horizontal">
        <TextBlock Background="Green" Text="some text" Width="200"/>
        <TextBox Width="{Binding ???}" />
        <TextBlock Background="Red" Text="some text" Width="50"/>
    </StackPanel>
</Window>

如何绑定第二个文本框的宽度以填充剩余空间?

谢谢。