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 -->