DTD for maxima internal form



Stavros Macrackis wrote, in part:

> What a DTD/Schema allows you to do is to specify restrictions
> on structures, so as to disallow -- at read time -- [illegal
> constructions.] There is no standard way to do this in Lisp. 
> You may or may not consider  this useful, but it is *not* the
> same thing as the Read function.

I agree that checking against a grammar is a useful function.
If that's what we want, there's no need to drag in XML -- 
I'm pretty sure that one could discover/invent equivalent
functionality, as applied to s-expressions, without much trouble.
I have looked into this a little bit, and the it appears that
some of the code in cl-xml could be adapted for that purpose,
but I am hampered by my own weak Lisp skills.

For what it's worth,
Robert Dodier

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