In general, DTD customization layers look like this:
<!-- Turn off markup --> <!-- Redefine parameter entities --> <!-- include the base DTD --> <!ENTITY % docbook PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN" "http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd" > %docbook; <!-- Redefine elements/attributes -->