Substitution groups: Is the "or" in "restriction or extension" ex clusive?

I understand that the type of a substitution element must be derived from
the type of the substitution group's head element. And that "derived from,"
as mentioned in the Primer, is elaborated to mean "the same as the head's
type definition or restrictions or extensions of it." 

		2.2.2.2 Element Substitution Group
<http://www.w3.org/TR/2001/REC-xmlschema-1-20010502/> 
		...
		All such members must have type definitions which are either
the same as the head's type definition or restrictions or extensions of it.

My question is, is that "or" in "restrictions or extensions" an exclusive
or, for all subsequent derivations down the chain?

For example, if I use the common practice of performing derivation by
extension and derivation by restriction in separate steps, can the result of
such a two-step derivation be the legal type of a substitution group member?

Would the following be legal?

	SH  of type A (substitution group head)
	type B is derived by restriction from A
		type C is derived by extension from B
	SM of type  C is a member of substitution group SH 

Thanks,

Mark


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Mark Feblowitz                                   [t] 617.715.7231
Frictionless Commerce Incorporated     [f] 617.495.0188 
XML Architect                                     [e]
mfeblowitz@frictionless.com
400 Technology Square, 9th Floor 
Cambridge, MA 02139 
www.frictionless.com

Received on Thursday, 10 January 2002 16:18:40 UTC