anchor.diff:  
   %% Defines how often an optimization pass happens.
   -define(CONTRACTION_INTERVAL, 1000).
  +
  +%% Initial depth of all nodes.
  +-define(BASE_DEPTH, 0).
   
   %%%===================================================================
   %%% Type definitions
  
positive.code:     {reply, {ok, Edges}, State};
  
  handle_call({add_vertices, Vs}, _From, #state{dag=Dag}=State) ->
      [digraph:add_vertex(Dag, V, 0) || V <- Vs],
      {reply, ok, State};
  
  handle_call(to_dot, _From, #state{dag=Dag}=State) ->
  
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(_, _) ->
  