RE: Understanding Restrictions on Groups

> The motivation is to provide an invariant rather like that of 
> object-oriented programming languages:  any valid member of a 
> type defined by restriction is also a valid member of the 
> type defined by its base.

I think that what this demonstrates is that's there's a set of use cases
where what people want is "a schema that's the same as X except for
difference D", with no constraints on the nature of the difference D - it
might be making a mandatory element optional, or adding values to an
enumeration, or changing something from float to double.

People imagine that's what xs:redefine was designed for, and perhaps it
should have been. Rather like overriding in XSLT, come to think of it - no
constraints requiring compatibility with whatever it is you are overriding.

Michael Kay
http://www.saxonica.com/