anchor.diff:      [{users, []},
        {sources, []},
        {grants, []},
  -     {revokes, []}
  +     {revokes, []},
  +     {epoch, os:timestamp()}
       ].
   
   get_meta() ->
  
positive.code:             %% add a source for each user
              case add_revoke_int(UserList, Bucket, Revokes, Meta) of
                  {ok, NewMeta} ->
                      put_meta(NewMeta),
                      ok;
                  Error2 ->
                      Error2
  
negative.code:     Meta = get_meta(),
      case validate_permissions(Grants) of
          ok ->
              put_meta(stash(epoch, {epoch, os:timestamp()},
                             add_grant_int([all], Bucket, Grants, Meta)));
          Error ->
              Error
      end;
  