p4raw-id: //depot/perl@16179
This should be allowed if the new keyset is a subset of the old
keyset. May require more extra code than we'd like in pp_aassign.
+=head2 Should overload be inheritable?
+
+Should overload be 'contagious' through @ISA so that derived classes
+would inherit their base classes' overload definitions? What to do
+in case of overload conflicts?
+
=head1 Vague ideas
Ideas which have been discussed, and which may or may not happen.