anchor.diff:  add_edges(Src, Dst, Pid) ->
       gen_server:call(?MODULE, {add_edges, Src, Dst, Pid}, infinity).
   
  +n_vertices() ->
  +    gen_server:call(?MODULE, n_vertices, infinity).
  +
  +n_edges() ->
  +    gen_server:call(?MODULE, n_edges, infinity).
  +
  +in_degree(V) ->
  +    gen_server:call(?MODULE, {in_degree, V}, infinity).
  +
  +out_degree(V) ->
  +    gen_server:call(?MODULE, {out_degree, V}, infinity).
  +
  +out_edges(V) ->
  +    gen_server:call(?MODULE, {out_edges, V}, infinity).
  +
  +in_edges(V) ->
  +    gen_server:call(?MODULE, {in_edges, V}, infinity).
  +
   %%%===================================================================
   %%% gen_server callbacks
   %%%===================================================================
  
positive.code: %% @private
  -spec handle_call(term(), {pid(), term()}, #state{}) ->
      {reply, term(), #state{}}.
  
  handle_call({add_vertex, V}, _From, #state{dag=Dag}=State) ->
      digraph:add_vertex(Dag, V),
  
negative.code:          add_edges/3,
           add_vertex/1,
           add_vertices/1]).
  
  %% Test
  -export([n_vertices/0,
           n_edges/0,
           out_degree/1,
           in_degree/1,
           out_edges/1,
           in_edges/1]).
  
  %% gen_server callbacks
  -export([init/1,
  