Alternatives to redefine

Hi

I am developing a schema that must be extensible for different
applications/purposes.  One sub-tree of the schema is the variable
part.  A <results> node may contain a sequence of other elements
but the "other" elements is what varies from one appplication to the
next. 

For example, one application may have results of type:
  A, B, C
while another applicaiton has
  A, D, E

<results> nodes may appear several times within an instance
but a <results> node always has a sequence of one of the
above allowable types (they are never mixed)

e.g., 
<results><A/><A/></results>
<results><C/></results>

are valid but
<results><A/><B/><B/></results>
is not

I am trying to define a basic schema and enable each applicaiton
to extend it in order to define the allowable result types.  

I have been experimenting with using a common base class, and
substitution group, and also redefine.

The other requirement I have is I want to be able to use Castor
to generate corresponding java classes.  When I tried using <xsd:redefine>
I discovered that Castor does not pick up the substitution groups
correctly.

I would appreciate any ideas or suggestions.
Bowden
wiseb@acm.org