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Subject: Re: [ubl] Substitution Groups - Use 'em or lose 'em?

Thanks for responding Dan. I thought that one of the good features of substitution groups is that you can always substitute for a global element. However, it is required to declare your substitution so that it is always explicit what the content model is. The way we have proposed the use of substitution groups, a schema extension explicitly declares the substitutions made. In the instance document the substitutions are delineated by the custom name space qualifier that preceded the standard un-substituted name.
Also, we have not been able to make redefine work in this application due to the following:
1) UBL document schemas import code list schemas. Therefore in a derived schema (that can't alter either the ubl schemas or the underlying code lists) errors are generated because the unredefined schemas had to be imported into the document schemas. We have tried various schemes to overcome this. Can you make it work? Remember, no changes in the code lists or the ubl schemas, only in the custom schemas.
2) Redefine can't extend an enumerated code list, only restrict it. This means that the base definition has to be non-enumerated. Then you can't validate against the standard code list without redefining to it.
If a redefine solution could work this would be most ideal. Until then, I favor substitution groups.
In a message dated 8/29/2005 7:09:22 P.M. Eastern Daylight Time, DVINT@acord.org writes:

I do not like using them because there is no way to know what the final content model is without some tool support. What I don’t like is that looking at the content model I can’t tell that a substitution is allowed. I have to track down all the elements that can be substituted, without there being some flag on the head element to indicate substitutions are being made. Sort of a good and bad point about them, is they also require all the substituted values to be based upon the same type, this is not always a good requirement.


Instead of this, ACORD has decided to live with the minor namespace issues associated with redefine and prefer this approach for extensions and restrictions.




From: Burnsmarty@aol.com [mailto:Burnsmarty@aol.com]
Sent: Monday, August 29, 2005 3:40 PM
To: ubl@lists.oasis-open.org
Subject: [ubl] Substitution Groups - Use 'em or lose 'em?




We are working on extensibility models for standardized business exchange
schemas. These schemas, under development by several standards organizations
make extensive use of hierarchical schemas and namespaces, some of which
include schemas developed by third parties.

When a user or user community seeks to use these schemas, and, needs to
modify them in some way (without altering the underlying standardized
schemas), substitution groups can be a powerful and explicit mechanism for
such extensions and restrictions.

Several participants in these standard schema efforts have expressed reserve
from utilizing the W3C mechanism of substitution groups due to their
experience with non-uniform support of parsers for this schema feature.

In your experience, what are the concerns or recommendations on the
incorporation of substitution groups into the naming and design rules of
standardized schemas?

Should substitution groups be relied upon as an extension mechanism?

Marty Burns
Hypertek, Inc.
P +1(301)315-9101
E burnsmarty@aol.com



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