anchor.diff:                          ok;
                       Depth0 ->
                           VertexResult = digraph:add_vertex(Dag, Dst, Depth),
  -                        lager:info("Vertex: ~p re-created with depth ~p =>~p; result: ~p",
  +                        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)])
  
positive.code: 
  -spec handle_call(term(), {pid(), term()}, #state{}) ->
      {reply, term(), #state{}}.
  
  handle_call(n_vertices, _From, #state{dag=Dag}=State) ->
      {reply, {ok, digraph:no_vertices(Dag)}, State};
  
negative.code: %% API
  -export([start_link/0,
           will_form_cycle/2,
           vertices/0,
           add_edges/6,
           add_vertex/1,
           add_vertices/1]).
  