xml-dev - required data

[Craig Miller <craigm@roninkuma.com>]

I currently have a dtd as follows:

<?xml version="1.0" encoding="UTF-8"?>
<!ELEMENT authentication (sessionid, serialnumber, username)>
	<!ELEMENT sessionid (#PCDATA)>
	<!ELEMENT serialnumber (#PCDATA)>
	<!ELEMENT username (#PCDATA)>

The xml file looks like:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE authentication SYSTEM "auth.dtd">
<authentication>
	<sessionid>12345</sessionid>
	<serialnumber>67890</serialnumber>
	<username>Hello</username>
</authentication>

I was wondering if there is a way to make the data required, not just the
tag.  For example, if I remove <sessionid>12345</sessionid> from the xml
file then it fails, as it should.  I would however like it to fail if the
tag looks like this also <sessionid></sessionid>.  Is there a way to mark
the dtd to fail if the value is NULL.

Thanks for any help you can provide,

Craig