newbie-ish DTD question: am i asking too much of XML, or of my brain?

[Rick Wayne]

hi all,

i have an astoundingly simple document format that i want to define via
a DTD, and can't get the parsers to let me do it.  so far i've looked in
_the_XML_companion_, _the_XML_black_book_, and perused the spec.  my
ignorance still throbs mightily.

in english:

i want my documents to consist of a "model" element.  a model can
contain "topics" and "links", any number, in any order.  in turn, topics
can contain topics and links, in any number, in any order.  i can
tighten up the rules a little if need be -- for instance, requiring that
models contain at least one topic.  but because these represent
mathematical operations, i can't just require that the topics come first
and the links follow (e.g., <!ELEMENT model(topic*, link*)>.

in (attempted) XML, with ATTLIST tags removed:

<?XML version="1.0" encoding="UTF-8" standalone="no"?>
<!ELEMENT model (topic* link*)>
<!ELEMENT topic (topic* link*)>
<!ELEMENT link>

all non-profane feedback gratefully accepted.

thanks for taking the time to have a look.  if you wish to respond via
private email instead of replicating to the list, that's dandy-fino with
me.  i doubt this problem is sufficiently generic to interest everyone.

rw


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