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   RE: Again wit da AND and Repetitions

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  • From: "Robert C. Lyons" <boblyons@unidex.com>
  • To: XML Dev <xml-dev@ic.ac.uk>
  • Date: Fri, 14 May 1999 10:19:09 -0400

John Cowan wrote:
     "IIRC the canonical way of doing (A & B & C) is to transform it
into (A | B | C)* and then do a post-check that each of A, B, C
appears exactly once.  ..."

It seems that this approach would require the parser to perform look ahead 
for the following content model:
( (A & B & C), C, C, C )

This content model would be transformed into: ( (A | B | C)*, C, C, C )

The transformed content model is non-deterministic. For example, the 
parser would have to look ahead when parsing the first C in the following 
    C B A C C C

The original content model is deterministic; the parser would not have to
look ahead to determine that the first C (in C B A C C C) matches 
the (A & B & C) portion of the original content model.


Bob Lyons
EC Consultant
Unidex Inc.

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