This line declares a XML namespace for the objects that are inside the file. It works as any other similar declaration in your other XAML files. To fix it:
where <namespace> should be the namespace where the MenuModel and MenuItem classes are declared; <assembly> should be the assembly which contains those classes.
In our example the namespace is Examples.Menu.Common (see the MenuItem.cs for example) and the assembly is named Examples.Menu (this is set in the project properties).
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