Hi, Lynn...
I don't know that I've ever had to add XInclude elements to a DTD, though
I'm pretty sure I've had to do so for other namespaced elements.
The trick is that DTD, unlike Schema, is not namespace-aware, so you really
just need to decide what prefix you'll use and declare the element with
that as part of the name and add the appropriate attribute definitions,
including the one for the namespace declaration (i.e., "xmlns:xi"). This
all works because the colon is a valid XML name character. As far as DTD
is concerned, it's just another name.
-Brandon 🙂