XML Schema 'all' question

Hello,

If the following question should be discussed elsewhere please give me
a hint. Thanks.

I try to define a modular XML schema using inheritance with the
"xs:extension" mechanism.

I would like to implement such an inherited type extended by an xs:all
model group. But that fails at the "cos-all-limited" constraint .

      http://www.w3.org/TR/xmlschema-1/#cos-all-limited

Which valid alternatives exists to my non-valid implementation? What
would be the best workaround for this issue?

I attach a shortened generalised example to show my intention.

Kind regards...
Susanne