Question on access levels for blocks

At the moment you can only specify one group per access level. So how do
you specify that e.g. site managers + administrators can modify a block?

This would imply that:
- either we need to specify that all admins also belong to site
   managers (which changes how groups are used),
- or that we should be able to specify several groups per access level
- or perhaps vice-versa, to specify several access levels per group.

Either way, I seem to be missing something here ;-)

Mike.