anchor.diff:  -type group_privilege() :: group_change_data | group_invite_user |
   group_remove_user | group_join_space | group_create_space |
   group_set_privileges | group_remove | group_leave_space |
  -group_view_data | group_create_space_token.
  +group_view_data | group_create_space_token |
  +group_join_group | group_create_group_token | group_remove_group.
   
   %%%===================================================================
   %%% API
  
positive.code:         group_user(),
          ordsets:from_list([
              group_invite_user,
              group_remove_user
          ])
      ).
  
  
negative.code:             group_leave_space,
              group_set_privileges,
              group_change_data,
              group_remove,
              group_join_group
          ])
      ).
  
  