anchor.diff:  -spec add(node(), non_neg_integer()) -> ok.
   add(Node, Count) ->
       gen_server:cast({?SERVER, Node}, {add, Count}).
  +
  +-spec add(node(), non_neg_integer(), proplists:proplist()) -> ok.
  +add(Node, Count, Opts) ->
  +    gen_server:cast({?SERVER, Node}, {add, Count, Opts}).
   
   -spec remove(non_neg_integer()) -> ok.
   remove(Count) ->
  
positive.code: %% ------------------------------------------------------------------
  %% callbacks handlers
  %% ------------------------------------------------------------------
  -spec handle_add(non_neg_integer(), state()) -> ok | list({ok, pid()}).
  handle_add(_Count, #state{scenario=undefined}) ->
      lager:error("add users invoked, but no scenario defined");
  handle_add(Count, #state{scenario=Scenario,
                           scenario_state=State,
                           nodes = Nodes,
                           node_id = NodeId}) when
        is_integer(Count), Count > 0 ->
      Last = case ets:last(?TABLE) of
                 '$end_of_table' -> 0;
                 Other -> Other
             end,
      UserIds = node_userids(Last+1, Last+Count, Nodes, NodeId),
      start_users(Scenario, UserIds, State).
  
  -spec handle_remove(non_neg_integer(), amoc:remove_opts(), state()) -> ok.
  handle_remove(_Count, _Opts, #state{scenario=undefined}) ->
  
negative.code:             application:stop(amoc)
      end.
  
  -spec start_users(amoc:scenario(), [amoc_scenario:user_id()], non_neg_integer(),
                    state()) -> [term()].
  start_users(Scenario, UserIds, Interarrival, State) ->
      [ start_user(Scenario, Id, Interarrival, State) || Id <- UserIds ].
  
  -spec start_user(amoc:scenario(), amoc_scenario:user_id(), non_neg_integer(),
                   state()) -> supervisor:startchild_ret().
  start_user(Scenario, Id, Interarrival, State) ->
      R = supervisor:start_child(amoc_users_sup, [Scenario, Id, State]),
      timer:sleep(Interarrival),
      R.
  
  -spec stop_users([amoc_scenario:user_id()], boolean()) -> [true | stop].
  