anchor.diff:  
       ?assertMatch(Pid when is_pid(Pid), ServerPid),
   
  -    stop().
  +    {ok, stopped} = stop().
   
   sum_areas_test() ->
       Shapes = [{circle, 3}, {rectangle, 4, 6}],
  
positive.code: 
      ?assertEqual({ok, 52.27433388230814}, Result),
  
      stop().
  
  stop_test() ->
      {ok, Pid} = start(),
  
      {ok, stopped} = stop(),
  
      receive after 10 -> ok end,
      ?assertNot(is_process_alive(Pid)),
  
      stop().
  
  sum_unknown_areas_test() ->
      {ok, _Pid} = start(),
  
negative.code: 
      ?assertMatch({error, {function_clause, _Detail}}, Result),
  
      {ok, stopped} = stop().
  
  never_die_test() ->
      {ok, Pid} = start(),
  