SQC does not allow xs:all in xs:extension, is that right?   

The XMLSpy and the XSV
(ftp://ftp.cogsci.ed.ac.uk/pub/XSV/XSV210.EXE)
both allow this. For example, for the following schema, sqc will report an error. But I think it's valid.

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="unqualified">
<xs:element name="Address" type="Address" abstract="true"/>
<xs:complexType name="Address" abstract="true">
<xs:all>
<xs:element name="name" type="xs:string"/>
<xs:element name="street" type="xs:string"/>
<xs:element name="city" type="xs:string"/>
</xs:all>
</xs:complexType>
<xs:element name="USAddress" type="USAddress" substitutionGroup="Address"/>
<xs:complexType name="USAddress">
<xs:complexContent>
<xs:extension base="Address">
<xs:all>
<xs:element name="state" type="xs:string"/>
<xs:element name="zip" type="xs:positiveInteger"/>
</xs:all>
</xs:extension>
</xs:complexContent>
</xs:complexType>
</xs:schema>