XInclude, glossaries within ‘set’

Hello,

1. This is apparently invalid as ‘xi:include’ is not allowed within ‘set’:

   ,----
   | <?xml version="1.0"?>
   | <set xmlns="http://docbook.org/ns/docbook"
   |      xmlns:xi="http://www.w3.org/2001/XInclude"
   |      xml:lang="en"
   |      version="5.0">
   |   <info>
   |     <title>Set title</title>
   |   </info>
   |   <xi:include href="book1.xml"/>
   |   <xi:include href="book2.xml"/>
   | </set>
   `----

   Is this intentional?  What is the rationale?

2. Is there a way to provide a set-wide glossary?  I see that
glossaries can be placed within books, but not within sets.

Aankhen