Importing external XML documents into an XML doc?

Hi,

According to http://www.w3.org/TR/1999/REC-xslt-19991116#function-document ,
we're allowed to access XML docs other than the main source doc. I assume
that this means we can import an external set of nodes into the current node
tree? How would this be implemented? Searched around for examples but
haven't been able to find any so far. And I've been lousy at understanding
the W3C doc (I need real life samples, sigh).

This feature would be useful 'cos I would be able to segregate common info
such as a website's main menu into a common file w/o maintaining a different
set in each XML file. Yeah, I could do this at a separate layer such as the
PHP engine generating the XML files but for a simple static site I would
prefer to do this non-programatically.

Would appreciate any pointers on this, thanks in advance!
Wong