[log in to unmask] writes

> If I inlcude some tags from--say--TEI Lite, which is SGML and does not
> have close tags for--say--<p>,

But TEI Lite _does_ have end-tags for <P>. It's just that using the
end-tag is optional. Perhaps a better example would be PTR or INDEX

> will that mean that my xml document is not well-formed?

Yes. In XML all elements must have start- and end-tags unless they
are declared as EMPTY and expressed as <PTR/> (the alternative being
<PTR></PTR> and never having any content).

> Or will my closing of the xmlns: tag that is ref'd to TEI
> serve to be "well-formed"?

Nope. There is no mechanism defined to make or allow an XML processor
to read an SGML DTD and work out that <tei:PTR> in your XML document
actually means <tei:PTR/>. Be nice if there was, but it would have
meant washing out all the shortcuts that XML makes and forcing it to
include all the original SGML stuff that was removed which made it
XML...just so it could read SGML DTDs.

The easy way is to use the XML version of TEI Lite (Patrice Bonhomme)
and copy the declarations from there into your new DTD.