anchor.diff:  %%      as adding the same vertex multiple times removes any
   %%      metadata (labels).
   %%
  --spec add_if_pid(digraph:graph(), pid()) -> ok.
  +-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);
  
positive.code:     gen_server:call(?MODULE, {add_vertices, Vs}, infinity).
  
  %% @doc Check if linking the given vertices will form a loop.
  -spec will_form_cycle(list(id()), id()) -> boolean().
  will_form_cycle(Src, Dst) ->
      gen_server:call(?MODULE, {will_form_cycle, Src, Dst}, infinity).
  
  
negative.code:                     transform :: function(),
                      write :: function()}).
  
  -type lasp_vertex() :: id() | pid().
  
  %% Return type of digraph:edge/2
  -type lasp_edge() :: {digraph:edge(),
                        digraph:vertex(),
                        digraph:vertex(),
                        #edge_label{}}.
  
  %%%===================================================================
  %%% API
  