Defining an XML Schema using multiple namespaces?

[Jan Algermissen <jalgermissen@topicmapping.com>]

Hi,

I am having trouble spotting documentation for the XSD question  
below, can anyone point me to the right part of the specs or example  
schemas?

The issue:
Suppose I have two XML namespaces, one defining core concepts such as  
'Address' and the other one defining things more at a business level,  
for example 'PurchaseOrder'. Both are just namespaces, defining the  
concepts not schemas defining any content models.

Using concepts from those two namespaces, I now want to assemble an  
XML schema to express content models for the concepts in the  
namespaces (and I also want to mix in xlink attributes).

For example, I might want to say that a bo:Person must have a  
core:Address child and that it must have a bo:Parent child that must  
have an xlink:href attribute.

I there a way to do this (especially using element names from  
different namespaces)?


(AFAIK the xlink attributes would be done with allowing any attribute  
from xlink, is that correct? Can one be more constraining here?)


Many thanks in advance!

Jan