Re: support for substitution groups, support for redefines?

> That doesn't sound like much of a justification. Why would I want to define
> a set of rules that documents must conform to, and then allow the author of
> a document to modify those rules for his particular document?

Great question-- but that is not at all what I am arguing for. What I am 
arguing for is the extension of an existing language by a second author 
of a second set of rules. The redefined schema would still be a defined 
set of rules for a class of documents (i.e. instances) that would be 
seen as the one "defined set of rules" from the document author's 
perspective.

The choice of which schema to use is still very much a business decision 
in many of the environments that I work-- so you will use whatever the 
business people say. In the mortgage industry this amounts to using the 
core set of standards defined by MISMO and any extensions for a 
particular trading partner. Validating that the partner has implemented 
those extensions correctly is an excellent justification.

All the best,
Jeff Rafter