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   Re: various issues

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  • From: David Megginson <dmeggins@uottawa.ca>
  • To: Peter@ursus.demon.co.uk
  • Date: Fri, 11 Apr 1997 09:10:51 -0400

Peter Murray-Rust writes:

 > What follows is the meta-DTD, right?  And the system knows it's
 > a meta-DTD because of the inclusion of one or more <?ArcBase PIs?

No, it's a derived DTD (actually, a DTD for a document that implements
a derived architecture).  A meta-DTD will not usually have an
<?ArcBase ..?> PI, unless it is mapped to yet another meta-DTD.

If you write a DTD, and include

  <?ArcBase html?>

in that DTD, you are suggesting that your DTD uses HTML as a base
architecture.  The HTML DTD is the meta-DTD, and it is not modified in
any way.

 > What this is doing is mapping elements in different DTDs to the
 > same element in your meta-DTD.  This implies that they are synonyms
 > in some way.  Does it assume that they have the same content model,
 > attributes and that the elements in the content model have
 > (recursively) been mapped onto the DTD.  Or are you essentially
 > aliasing SYM in HTML to SYMBOL and SYMBOL in MATH to SYMBOL but
 > having to provide different content models and processing for each?

No, it's the other way around -- the DTD for the derived architecture
maps some of its elements to those in one or more meta-DTDs (though
you don't have to do the mapping in the DTD itself).  If you're
creating document type X, and you're including architectures from
CALS, HTML, and TEI-L, then you put the mappings in the X DTD.

They don't have to have exactly the same content models -- the content
model in the derived DTD can be more restrictive (but not less).

 > This means your DTD is a superset of the other DTDs and your own elements?

Not exactly -- it means that there is an intersection between the
semantics of your DTD and the semantics of each of the meta-DTD.  Your
DTD may have element types that are not in the meta-DTD, and the
meta-DTD may have element types that are not in your DTD.

 > I think I start to understand.  By 'structure' you mean the tree structure
 > of each DTD?  In which case AFs are a way of providing multiple views of the
 > same document?

I don't follow.

All the best,


David Megginson                 ak117@freenet.carleton.ca
Microstar Software Ltd.         dmeggins@microstar.com
University of Ottawa            dmeggins@uottawa.ca

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