DTD and Illegal Construct

[Jim Shain <Jim.Shain@alltel.com>]

I wanted to run this past some knowledgeable people.  Is the following a legal
or illegal construct for an element in a DTD?

 <!ELEMENT Request ((A, B, C?) | (A?, B, C?, D))>

Some have stated that it is illegal because "When A is present it isn't possible
to determine whether or not D is required."

Jim Shain
Sr. C/S Architect
ALLTEL Information Services