Standalone nature of included documents

Hello,

I was wondering what an XInclude processor's behaviour should be when
included documents contain qualified element names which use undefined
namespace prefixes. For instance:

file1.xml:
<p:root xmlns:p="http://www.example.org" xmlns:xi
="http://www.w3.org/2001/XInclude">
    <xi:include href="file2.xml"/>
</p:root>

file2.xml:
<p:child/>

In the above case, file2.xml would not be valid on its own, because the
prefix 'p' is undefined, and the [in-scope namespaces] is empty (except for
the implicit prefix). But when it is included into file1.xml, it would be
valid.

Is there a defined behaviour for this? Is the [in-scope namespaces] of the
include parent combined with the [in-scope namespaces] of the top level
included elements?

Thanks,
Peter McCracken
IBM Toronto Lab