Inheritance in XML (was Re: Problems parsing XML)

[Paul Prescod]

Martin Bryan wrote:
> 
> 
> In SGML you can use exclusions to make an element a true subclass of
> another:
> 
> <!ELEMENT X  (%Y-contents;) -(a|b|c)>
> 
> providing a, b and c are optional components within the model for Y.

Element X is not a true subclass or subtype. Given a content model:

<!ELEMENT J (Y)>

You cannot use an X.

 What you've done above is make an element whose content model is more
restrictive than some other content model. You can also do that without
exclusions. I don't think I've ever used exclusions in that way. One big
problem is that the exclusion doesn't just change the content model, but
the content model of all of X's children. You don't want that if all you
need is content model subsetting.

 Paul Prescod  - http://itrc.uwaterloo.ca/~papresco

"Journalism is good if you follow the rules. Don't allow the human 
rights groups to spoil your profession" 
    - Col. Godwin Ugbo of the  Nigerian military dictatorship

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