redefine and multiple includes

Hello,

I searched through the archives but could not find a solution.

I am creating 3 levels of a schema. Level a is the basis. Level b includes
level a and redefines some groups and complexTypes. Level c includes level b
and adds some more elements to those already added in level b. However it
looks like the addition of level b are gone in level c, so I have to add
those elements that I added in level b again in level c. Is this correct?

schema level a:
<xs:group name="grp.on">
  <xs:sequence>
   <xs:element ref="description" minOccurs="0"/>
  </xs:sequence>
 </xs:group>

schema level b: redefine level a
<xs:redefine schemaLocation="level-a.xsd">
<xs:group name="grp.on">
   <xs:sequence>
    <xs:group ref="grp.on" minOccurs="0"/>
    <xs:element ref="var" minOccurs="0" maxOccurs="unbounded"/>
   </xs:sequence>
  </xs:group>


schema level c: redefine level b
<xs:redefine schemaLocation="level-b.xsd">
<xs:group name="grp.on">
   <xs:sequence>
    <xs:group ref="grp.on" minOccurs="0"/>
    <xs:element ref="not" />
   </xs:sequence>
  </xs:group>

In level c the group grp.on should contain description, var and not, but it
only contains description and not, unless I add var as well.

Thanks,

Francis Brouns