DTD question

[John Martin]

Greetings.  I'm new to this list and to XML.

In a DTD, can I base the inclusion of an optional element on an
attribute?  For example, I have:

<!ELEMENT NCSB-Message (Service-Version-Number, Service-Release-Number,
Service-Completion-Code?, Service-Reason-Code?)>
<!ATTLIST NCSB-Message MESSAGE-TYPE (REQUEST | RESPONSE) #REQUIRED>

Is there a way to indicate in the DTD that if the attribute
MESSAGE-TYPE=RESPONSE, then I want the elements Service-Completion-Code
and Service-Reason-Code to be required instead of optional?

Thanks.






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