open or extensible schemas

Hi all,

I'm trying to write an "open" schema, one that be extended by
third parties.  I've been reading Eric van der Vlist's "Writing
Extensible Schemas" [1] which details quite nicely how to write
extension modules, but it's not exactly what I'm looking for.
Is there a way to define a schema that rather than saying what
/can/ be present, just says what /must/ be present?  

For example, say these are required elements:

<foo>
  <bar/>
</foo>

Can can there be schema that would validate the following
document:

<foo>
  <bar/>
  <zonk/>
  <bonk/>
</foo>

because foo and bar are present?  Basically I want to say that
*anything* can be under element foo as long as bar is present.

Thanks,
Eric

[1] http://books.xmlschemata.org/relaxng/ch14s01.html