anchor.diff:      {ok, pid()}.
   start_link(Scenario, Id, State) ->
       proc_lib:start_link(?MODULE, init, [self(), Scenario, Id, State]).
  +
  +stop() -> throw(normal_user_stop).
   
   -spec init(pid(), amoc:scenario(), amoc_scenario:user_id(), state()) ->
       no_return().
  
positive.code: -spec flush_mailbox() -> ok.
  flush_mailbox() ->
      receive
          stop ->
              throw(stop);
          _ ->
              flush_mailbox()
      after 0 ->
  
negative.code:             end,
              normal
          catch
              throw:normal_user_stop ->
                  normal;
              E:Reason:Stacktrace ->
                  {E, {abnormal_exit, Reason}, Stacktrace}
          after
  