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Multiple rules in RELAX (was Re: And the DTD says...)

-- Caroline Clewlow wrote:
> Is this something that RELAX takes care of with it's <elementRule role=""
> label="">
> whereby two element rules can have the same role with a different label
> depending on how they constrain their child elements ?

RELAX Core allows the same label to be defined more than once.  
In DTDs, the first one wins.  In programming languages, the 
last one wins.  In RELAX Core, all of them win.

Here is an example of this feature.  Two modules in XHTML Basic 
define the parameter entity "head.content".

In xhtml-struct-1.mod

<!ENTITY % head.content
    "( %HeadOpts.mix;, %title.qname;, %HeadOpts.mix; )"

In xhtml-base-1.mod

<!ENTITY % head.content
    "( %HeadOpts.mix;,
     ( ( %title.qname;, %HeadOpts.mix;, ( %base.qname;, %HeadOpts.mix; )? )
     | ( %base.qname;, %HeadOpts.mix;, ( %title.qname;, %HeadOpts.mix; ))))"

When xhtml-base-1.mod is used, the dcl in xhtml-struct-1.mod is 

My rewrite of xhtml-struct-1.mod has a hedgeRule for head.content, 
and my rewrite of xhtml-base-1.mod also has another hedgeRule.
But the first one is never overshadowed.

As a result, the hedgeRule in the rewrite of xhtml-base-1.mod 
can be simplified.  It is equivalent to the following.

<!ENTITY % head.content
    "( %HeadOpts.mix;,
       ( ( %title.qname;, %HeadOpts.mix;, %base.qname;)
       | ( %base.qname;, %HeadOpts.mix;, %title.qname))

Although this feature is useful for modularization, I do not 
claim that we can get rid of features of <redefine>.  I 
hope to incorporate <redefine> feeatures in the upcoming 
ISO TR (not DTR) for RELAX Core.