anchor.diff:  
   -spec handle_call(term(), {pid(), term()}, #state{}) ->
       {reply, term(), #state{}}.
  +
  +handle_call(vertices, _From, #state{dag=Dag}=State) ->
  +    {reply, {ok, digraph:vertices(Dag)}, State};
   
   handle_call(n_vertices, _From, #state{dag=Dag}=State) ->
       {reply, {ok, digraph:no_vertices(Dag)}, State};
  
positive.code: %% API
  -export([start_link/0,
           will_form_cycle/2,
           add_edges/6,
           add_vertex/1,
           add_vertices/1]).
  
negative.code:             %% that's not there, we'll try to make an edge to a
              %% non-existent vertex and will trigger this
              %% error.  Ignore.
              {ok, State};
          false ->
              erlang:monitor(process, Pid),
  