xml-dev - RE: DDML Section 2.3.7: Nested SEQs within SEQ

[Ronald Bourret <rbourret@ito.tu-darmstadt.de>]

Amit Rekhi wrote:

> I was going thru Section 2.3.7 of the DDML spec and
> could not understand why the content model for 'Seq'
> <!ELEMENT Seq ((Choice | Ref | Model),(Choice | Ref | Model)+)>
>
> could not contain nested Seqs .
> for eg.
> Acc. to the above content model if
> I had an element decl like:
> <!ELEMENT Ele1 (A,B,(D,E,F)+,G,H)
> The DDML equivalent would be :
>
> <ElementDecl Name="Ele1">
>  <Model>
>     <Seq>
>        <Ref Name="A" />
>        <Ref Name="B" />
>        <Model>
>         <Seq Frequency="OneOrMore">
>           <Ref Name="D" />
>           <Ref Name="E" />
>           <Ref Name="F" />
>         </Seq>
>        </Model>
>        <Ref Name="G" />
>        <Ref Name="H" />
>     </Seq>
>  </Model>
> </ElementDecl>
>
> Why is something like the following
> DDML representation not allowed
> (minus the nested <Model>)?
>
> <ElementDecl Name="Ele1">
>  <Model>
>     <Seq>
>        <Ref Name="A" />
>        <Ref Name="B" />
>         <Seq Frequency="OneOrMore">
>           <Ref Name="D" />
>           <Ref Name="E" />
>           <Ref Name="F" />
>         </Seq>
>        <Ref Name="G" />
>        <Ref Name="H" />
>     </Seq>
>  </Model>
> </ElementDecl>

This is a good question and unrelated to the use of Model -- I looked at 
the oldest version of the DDML DTD that I have and it does not allow Seq to 
be nested inside Seq. Our original thinking was that we didn't feel the 
need to represent cases such as:

   <!ELEMENT A (B, (C,D), E)>

since this resolves to:

   <!ELEMENT A (B, C, D, E)>

However, this doesn't take into account the case you show. My best guess is 
that we simply didn't think of it.

-- Ron Bourret


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