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   RE: Request for help understanding the XML Recommendation

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  • From: "DuCharme, Robert" <Robert.DuCharme@moodys.com>
  • To: "'Michael Champion'" <Mike.Champion@softwareag-usa.com>, xml-dev@xml.org
  • Date: Fri, 28 Apr 2000 18:41:47 -0400

>Six of the eighty-nine productions are unreachable
>from the start symbol.
>The real problem is that one is *not* used at all, as
>far as I can tell.
>It is:
>  [30] extSubset    

>From a practical standpoint, it makes sense that extSubset isn't on the
right hand side of any production that leads back to the 'document' start
symbol: a document is an entity, and an extSubset is another entity that is
external to the document entity. 

>To a country boy like me, this means that the XML 
>Spec would have the same meaning if this
>production was entirely omitted. 

When a parser follows a PEReference to the entity outside of the document
entity, it should have some rules that let it determine whether the
referenced external entity is kosher or not, so the spec needs the extSubset
production to show the structure of an external subset. To do this, the
extSubset production references other productions in the grammar, which is
why it wouldn't be expressed as a separate grammar.

 My old CS textbooks aren't in the office where I'm typing this, so I can't
double-check whether multiple start symbols are allowed in a formal grammar,
but to me, the situation described above points to extSubset having start
symbol status itself. This is how it can play a necessary role in the spec
without being referenced by the right hand side of any other production.
And, in bottom-up parsing, once you've determined that an entity is an
external subset, you don't need any more productions for further parsing,
which would make that production's left-hand side the start symbol of that
parse, right? I could be wrong, because I've never written a parser outside
of school exercises, so I've never had to confront much of this face to

Bob DuCharme          www.snee.com/bob           <bob@  
snee.com>  "The elements be kind to thee, and make thy
spirits all of comfort!" Anthony and Cleopatra, III ii

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