Introducing computed pages!
<P@ComputedPage name="pgSub2" value="p2" @P>
This will automagically retrieve a page called p2 from the design collection and insert it in the current page (in place of the tag). In your action code, just add:
Which will cause the server to insert page p3 in that spot instead.
This is very useful for hiding or showing navigation panels etc based on user's rights, eg:
.... set to an empty string to not show any page at all