anchor.diff:                      ?log_info("~p: killed last vbucket (~p) for destination ~p", [SrcNode, VBucket, DstNode]),
                       ok;
                   _ ->
  -                    {ok, _} = start_child(SrcNode, Bucket, VBuckets -- [VBucket],
  +                    {ok, _} = start_child(SrcNode, Bucket,
  +                                          ordsets:del_element(VBucket, VBuckets),
                                             DstNode)
               end
       end.
  
positive.code:                 [VBucket];
              #new_child_id{vbuckets=Vs} = Child ->
                  kill_child(DstNode, Bucket, Child),
                  [VBucket|Vs]
          end,
      {ok, _} = start_child(SrcNode, Bucket, VBuckets, DstNode).
  
  
negative.code: 
  -include("ns_common.hrl").
  
  %% identifier of ns_vbm_sup childs in supervisors. NOTE: vbuckets
  %% field is sorted. We could use ordsets::ordset() type, but we also
  %% want to underline that vbuckets field is never empty.
  -record(new_child_id, {vbuckets::[vbucket_id(), ...],
                         src_node::node()}).
  
  