Why doesn't XML have Bag?

Paul Janssens paul.janssens at skynet.be
Fri Mar 26 15:15:34 GMT 1999

Lars Marius Garshol wrote:
> It is a limitation of DTDs and was introduced because without this
> operator element content models are easily mapped to finite state
> automatons, but the introduction of the '&' separator makes automaton
> generation much more difficult.
> Existing SGML parsers already do this, and there are some research
> papers giving algorithms for this, but the designers felt that this
> was one of the things that would have to go in the simplification from
> SGML to XML.

Please correct me if I am wrong here but isn't that trivial?
(you may get a BIG automaton, but it's not difficult to generate)

X -> A & B & C ;

can be expressed as

X -> A X_A | B X_B | C X_C ;

X_A -> B X_AB | C X_AC ;
X_B -> A X_AB | C X_BC ;
X_C -> A X_AC | B X_BC ;

X_AB -> C X_ABC ;
X_BC -> A X_ABC ;
X_AC -> B X_ABC ;

X_ABC -> ;

and it's 'easily' visualised by the number of possible shortest paths
between two opposing points on a hypercube.

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