DOCBOOK-APPS: Validate XML documents within Emacs

How can I set the XML_CATALOG_FILES to XML catalog file within Emacs 
such that I can evaliate(C-c C-v) XML documents using OASIS XML 
Catalogs(XML syntax catalog files)? There's no problems using 
SGML_CATALOG_FILES with TR9401:1997 syntax.

Sidney Lu
Academina Sinica Computing Centre