Proposition: "SGML is Gumming Up the Works"

Paul Prescod papresco at
Tue Sep 15 03:50:06 BST 1998

On 14 Sep 1998 matt at wrote:

> > What does it mean to "subclass" the PAREN element type when it is clearly 
> > used in two different contexts with two different content models? The 
> > answer: there is no PAREN type, really. There is a PAREN "tag" that can 
> > be used in completely different ways in completely different contexts.
> > 
> Why would anyone put a paren around args?  Args is already a grouping
> construct - paren is redundant there.  In the second case, wouldn't you
> rather use <EXPRESSION> than <PAREN>?  It always seemed to me that the
> elements of the DTD should sit at least one level above lexing, but PAREN
> is something the lexer does away with.  And doesn't it seem that ARGS and
> EXPRESSION are subclasses of a parent grouping element?

I used PARENs to use an example of the same token being used for 
different things that people would be familiar with.

Ar ARGS and EXPRESSION logically subclasses of a parent grouping element? 
Sure, at some level. But they don't share a content model, and they don't 
necessarily share attributes, so at the tree validation level, they are 
not really related.

Tables and figures are also related as "block-level objects" (in many
DTDs), but also do not share a content model or attributes. This is why I
feel strongly that element type subclassing is quite different from
inheritance in documents, just as in OO. 

> Are you calling for the resurrection of SHORTREFS?  Content models should
> ideally address the abstract syntax tree.  Lexical constraints address
> content.  If you want to cross them, you need something like SHORTREFS (or
> BNF.

Sorry, I was speaking loosely. I'm more interested in constraints at the 
tree level than lexical constraints. But I don't see why you think that 
lexical constraints need something like SHORTREFS or BNF. What about 
regular expressions? What would be fundamentally wrong with something 
like this:


 Paul Prescod

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