Abstract types in substitution groups

I've been looking at a published schema that uses an abstract type in a 
substitution group.  i.e. something like:


<xs:complexType name='abstractType' abstract='true'>
    <xs:sequence/>
</xs:complexType>

<xs:element name='myElement' type='abstractType' 
substitutionGroup='sgroup'/>


To me this seems invalid.  Or do substitutionGroups have special permission 
to use abstract types?

Thanks for any help,

Pete.
--
=============================================
Pete Cordell
Tech-Know-Ware Ltd
for XML to C++ data binding visit
http://www.tech-know-ware.com/lmx
(or http://www.xml2cpp.com)
=============================================