anchor.diff:  
   handle_call(step, _, State) ->
       #state{timers=Timers} = State,
  -    Min = min_interval(Timers),
  -    NewTimers = subtract(Min, Timers),
  -    NewState = State#state{timers=NewTimers},
  -    {reply, Min, NewState};
  +    case Timers of
  +        [] ->
  +            {reply, 0, State};
  +        _ ->
  +            Min = min_interval(Timers),
  +            NewTimers = subtract(Min, Timers),
  +            NewState = State#state{timers=NewTimers},
  +            {reply, Min, NewState}
  +    end;
   
   handle_call(fire, _, State) ->
       #state{timers=Timers} = State,
  
positive.code:     ?assertEqual(1, ?timer:fire(Pid)),
      assertMessage(c).
  
  
  -endif.
  
negative.code:             ?assertEqual(Msg, make_ref())
      end.
  
  assertNoMessage() ->
      Ref = {no_message, make_ref()},
      self() ! Ref,
      receive
          Ref ->
              ?assert(true);
          Other ->
              ?assertEqual(Ref, Other)
      end.
  
  
  instant_send_test() ->
      {ok, Pid} = ?timer:start_link(instant),
  