Sequence <was> Access Languages ...

Mark L. Fussell fussellm at
Sat Nov 22 09:34:02 GMT 1997

Rick Jelliffe <ricko at> wrote:
> I think you miss what is perhaps *THE* most important thing that SGML 
> content models represent:  sequence.
> This is one of the essential distinguishing features of SGML.
> If I have 
> <p>Refer also to 
> <citation>
>         <title>XML draft</title>
>         <text>at</text>
>         <url></url>
> </citation> for more info.</p>
> then the sequence of elements and data in to citation element 
> are vitally critical.  Sequence is not an artifact of formatting,
> in many cases, but as intrinsic to the data as encapsulation
> and so on.  
[SNIP to possible Content Model]
>        <!ELEMENT citation   ( title, text, url)>

I think your example shows the opposite.  There is no information change 
between any of the orderings within the citation:  <title><text><url> vs. 
<title><url><text> vs. <url><title><text> etc.  You may consider the 
desired presentation and encoding order to be only the first but that 
would be a view onto the information and not a property of the 
information itself.  You could alternatively define an attribute that 
says citations look good in English in that particular order.  Or maybe 
the 'at' should be derived and the content model is simply:
         <!ELEMENT citation   ( title & url)>

This works well with your next example too:
>       <!ELEMENT citation   ( title, text, name, text, url )>
        <!ELEMENT citation   ( title & editor? & url )>

Depending on whether the editor is included or not, different text would 
be generated at presentation.  

The generated text could still be encoded in SGML but as separate 
    <editorPrefix>edited by</editorPrefix>

I am not saying sequence is unimportant, but I think SGML is overly 
focused on it (from an IM perspective) because it comes from a 
paper/linear background.  Information is rarely linear: it is only time 
that is, which has caused some media [and the humans who use them] to be 
(mostly) linear also.  It can be difficult to break that linear 
assumption when it doesn't apply if your tools keep reinforcing it.

mark.fussell at

  i   ChiMu Corporation      Architectures for Information
 h M   info at         Object-Oriented Information Systems
C   u         Architecture, Frameworks, and Mentoring

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