needs to be displayed with %vd, make v-strings (readonly) objects
(class "vstring"?) with a stringify overload.
+=head2 Allow restricted hash assignment
+
+Currently you're not allowed to assign to a restricted hash at all,
+even with the same keys.
+
+ %restricted = (foo => 42); # error
+
+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.
+
=head1 Vague ideas
Ideas which have been discussed, and which may or may not happen.