group-store: Disallow joining of nonexisting groups
Returns %.n upon requesting a join of group that does not exist
++ peek-group-join
|= [rid=resource =ship]
- =/ =group
- (~(gut by groups) rid *group)
+ =/ ugroup
+ (~(get by groups) rid)
+ ?~ ugroup
+ %.n
+ =* group u.ugroup
=* policy policy.group
?- -.policy
%invite
