anchor.diff:      {reply, {ok, Edges}, State};
   
   handle_call({add_vertices, Vs}, _From, #state{dag=Dag}=State) ->
  -    [digraph:add_vertex(Dag, V, 0) || V <- Vs],
  +    [digraph:add_vertex(Dag, V, ?BASE_DEPTH) || V <- Vs],
       {reply, ok, State};
   
   handle_call(to_dot, _From, #state{dag=Dag}=State) ->
  
positive.code:                                                                transform=TransFun,
                                                                 write=WriteFun}),
  
          %% Determine depth.
          Depth = depth(Dag, Dst, 0),
  
          %% Re-create destination vertex with new depth.
          case digraph:vertex(Dag, Dst) of
              {_, Label} ->
                  case Label of
                      Depth ->
                          %% Already correct depth; ignore.
                          ok;
                      Depth0 ->
                          VertexResult = digraph:add_vertex(Dag, Dst, Depth),
                          lager:info("Vertex: ~p re-created with depth ~p => ~p; result: ~p",
                                     [Dst, Depth0, Depth, VertexResult]),
                          lager:info("Vertex: ~p edges: ~p",
                                     [Dst, digraph:edges(Dag, Dst)])
                  end;
              _ ->
                  ok
  
negative.code: -spec add_if_pid(digraph:graph(), lasp_vertex()) -> ok.
  add_if_pid(Dag, Pid) when is_pid(Pid) ->
     case digraph:vertex(Dag, Pid) of
        false ->
             digraph:add_vertex(Dag, Pid);
        _ ->
             ok
     end;
  
  add_if_pid(_, _) ->
  