how to edit multi-part files?

I have a large document which is split into several parts which are
included into the master file using system entities.
editing the sub-files using nxml mode is a nightmare: all the entities
defined in a separate file and included in the master file are not
recognized and the constant unsuccessful revalidation makes editing very
slow.
So, how do I tell nxml that the XML file I am editing is actually
included in the master file and thus should be validated as its part?
Thanks!
-- 
Sam Steingold (http://sds.podval.org/) on Ubuntu 10.04 (lucid)
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