Identical production rules

Eric Bohlman ebohlman at
Tue Sep 28 03:01:38 BST 1999

On Mon, 27 Sep 1999, Pierre Morel wrote:

> Why the XML specs declare two production rules with identical structure ?
> [30]  extSubset ::=  TextDecl? extSubsetDecl
> and
> [79]  extPE ::=  TextDecl? extSubsetDecl
> I don't understand very well the purpose of having two identical production
> rules.

Neither extSubset nor extPE are used as non-terminals in any other
productions; rather they're referred to in the text in statement of the
form "foo is well-formed if it matches the production labelled bar."  It's
just a coincidence of implementation that external subsets and external
parameter entities have the same syntax, and the redundancy makes human
communication easier.  Tim Bray could probably tell us if the redundancy
were the result of an earlier draft specifying different syntaxes for the
two cases.

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