In Squid, selectors will be more than just symbols.  I'm considering
adding an attribute to Selector to indicate whether it returns a value
or not.  All subclass/polymorphic methods would have conform to this,
and all senders would not be allowed to get the value of a
"procedure-only" selector.  This will speed up returns a little since
only "function" methods will have to return something.

Cheers,
Anthony