xsd:redefine issue using xsd:include

Hi,

I have three schema files a.xsd, b.xsd, c.xsd within the same namespace.

a.xsd includes b.xsd (<xsd:include schemaLocation="b.xsd"/>)

c.xsd redefines some types from a.xsd (<xsd:redefine
schemaLocation="a.xsd">)

Since a.xsd is including b.xsd I was thinking that in c.xsd I could
redefine types from a.xsd also. I did that and xerces doesn't complain
at all. However, other tools like Stylus Studio, XML Spy, IBM Schema
Quality Checker do. Is this w3c valid? It's not clear to me reading the
spec. Or is it a tools issue?

Best Regards,
-Marc