ignoring elements in certain namespace in schema?

I hope some XML schema expert can give me advice on this one (it's
probably a silly question..) I've got a schema that describes a vanilla
XML file, say:

<x>
  <y>
    <z/>
  </y>
</x>

I've recently had to allow extension elements in there. They have
user-defined local names, so cannot be enumerated in advance, but have
to be in their own namespace:

<x xmlns:foo="http://...">
  <y>
    <foo:checkme/> 
    <z/>
  </y>
</x>

Is there any way to augment the schema to make it ignore all elements in
namespace foo? Should it even be checking that namespace since it's not
its target namespace ?

Thanks,
Christian