How do I databind to a DataContext further up the tree in Silverlight?

In Silverlight, I am creating a button for every item in an ObservableCollection. I've added an ICommand to handle this on the object which has the ObservableCollection. In the XAML, how do I get back up to this from one of the collection items?

LayoutRoot.DataContext is set to an instance of the following class:

public class MainViewModel
{
    public ICommand TestCommand { get; protected set; }

    public ObservableCollection<string> Test { get; protected set; }    

    public MainViewModel()
    {
        Test = new ObservableCollection<string>();
        Test.Add("Hello");
        TestCommand = new DelegateCommand(Test,开发者_高级运维 CanTest);
    }

    private void Test(object parameter)
    {
        Test.Add("Test text");
    }

    private bool CanTest(object parameter)
    {
        return true;
    }
}

And using it with this XAML:

<StackPanel x:Name="LayoutRoot" Background="White">
    <ItemsControl ItemsSource="{Binding Test}" />

    <Button Command="{Binding TestCommand}">Push Me</Button> <!-- I can access TestCommand when I bind to it here -->

    <ItemsControl ItemsSource="{Binding Test}">
        <ItemsControl.ItemTemplate>
            <DataTemplate>
                <Button Content="{Binding}" 
                        Command="{Binding Path=TestCommand, Source=?????}" <!-- But how do I get back to the TestCommand from here? -->
                        CommandParameter="{Binding}" />
            </DataTemplate>
        </ItemsControl.ItemTemplate>
    </ItemsControl>
</StackPanel>

You can bind to the name of the root element

<Button Command={Binding Path=DataContext.TestCommand, ElementName=LayoutRoot}" />