A DTD puzzle: Need clarification

[Jerry Howard]

Looking for some DTD clarification.

In my current DTD, I have the following element:
<!ELEMENT NAME ( FIRSTNAME?, LASTNAME? ) >


I am trying to strengthen my DTD to allow for either FIRST, LAST, or
both, but I don't want the name tag to contain an empty value. The DTD
above allows me to have this, but also allows for a NAME tag to be
empty.  I have tried the following statement below(along with several
variations), but several xml parsers (IBM, Sun, ...etc) have not
accepted it

<!ELEMENT NAME ( (FIRSTNAME, LASTNAME) | FIRSTNAME | LASTNAME)>

I have browsed through several XML and SGML books looking for some
assistance, but haven't found it.

Any hints?

Thanks,
Jerry Howard

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