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   Re: A Line in the Declarative Syntax Sand(Was: XML complexity, namespace

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  • From: "Rick Jelliffe" <ricko@allette.com.au>
  • To: "XML Developers' List" <xml-dev@ic.ac.uk>
  • Date: Wed, 24 Mar 1999 12:25:26 +1100

From: David Megginson <david@megginson.com>
 >You're modelling exactly the same information about the picture in
>both -- data attributes provide an alternative mechanism for modelling
>the information, but they do not allow you to represent anything that
>you could not represent without them.

Except that the SGML example gives the attributes as belonging to the
thing pointed to (the entity) and not the particular invocation.

In the XML version, there is nothing to say that the attributes belong
to the thing pointed to rather than at the invocation. For example, take
the common case of where the entity has a size (natural size) and the
element also has size attributes (scaled size).

Surely the equivalent XML to the SGML examples given is really:

     <photodef id="p1" href="pic1.png" notation="text/png" width="300"
    type="I point to some object called an NDATA entity"
    content-model="I must be empty"
    addressing="don't count me as an element when doing treeloc" />
     <photo href="#p1" />

(and perhaps the photo element should not be  simple link)

The information modelled does not only include the elements and
attributes but also the structure, and the fact that an entity is
labelled as an entity, which have different addressing rules. In the
absense of XML having conventions for the last three attributes, I dont
think one can say that one can model everying that SGML models using

Rick Jelliffe

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