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I want to be able to produce a list of categories that are sub categories of a specific category.

This gives me everything under the KB category in the main namespace:
{{#ask: [[Category:KB]] }}

Following the documentation for limiting results to a specific namespace, this gives me everything in the Category namespace, i.e. ALL categories:
{{#ask: [[:Category:+]] }}

But this gives me nothing:
{{#ask: [[Category:KB]] [[:Category:+]]}}

How else can one query "all pages in the Category namespace, that are in the KB category"?