anchor.diff:  
       ?assertMatch(Pid when is_pid(Pid), ServerPid),
       
  -    stop(ServerPid).
  +    {ok, stopped} = stop(ServerPid).
   
   sum_areas_test() ->
       {ok, Pid} = start(),
  
positive.code: 
      ?assertEqual({ok, 52.27433388230814}, Result),
  
      stop(Pid).
  
  stop_test() ->
      {ok, Pid} = start(),
  
      {ok, stopped} = stop(Pid),
  
      receive after 50 -> ok end,
      ?assertNot(is_process_alive(Pid)).
  
  sum_unknown_areas_test() ->
      {ok, Pid} = start(),
  
negative.code: 
      ?assertMatch({error, {function_clause, _Detail}}, Result),
  
      {ok, stopped} = stop(Pid).
  