all group composition

Hi!

Is it possible to compose unordered group models
using xml schema? I need to validate two elements
with a lot of common children but I prefer not
imposing any particular ordering on them. I would
like to factorize the 80% of common children into
an <all> group and then join them to the rest:

<xs:group name="common">
  <xs:all>
    ....
  </xs:all>
</xs:group>

<xs:group name="group1">
  <xs:all>
    <xs:group ref="common"/>
    ....
  </xs:all>
</xs:group>

<xs:group name="group2">
  <xs:all>
    <xs:group ref="common"/>
    ....
  </xs:all>
</xs:group>

Of course I can't do this because <all> must be top
level in the group and moreover it can't contain <group>.

Is it possible to achieve the factorization in another way?

Thank you in advance.
Regards,
Carlos.