" is a URI namespace that maps to multiple CLR namespaces. So if you have Controls and Input assemblies that using this URI namespace you will see all controls from both assemblies.
Using the old syntax is still supported but will limit you and you will have to add two namespaces to see controls from both assemblies.
We did this change with Silverlight 4 (before that URI namespaces were not supported for custom controls).
We will have to update our documentation.
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