anchor.diff:      {error, already_stopped} = stop(Pid).
   
   sum_areas_unknown_shapes_test() ->
  +    Shapes = [{ellipse, 3, 3}],
       {ok, Pid} = start(),
  -    Shapes = [{ellipse, 3, 3}],
   
       Result = sum_areas(Shapes, Pid),
   
  
positive.code:     {ok, stopped, _state} = stop(ServerPid).
  
  sum_areas_test() ->
      {ok, Pid} = start(),
      Shapes = [{circle, 3}, {rectangle, 4, 6}],
  
      Result = sum_areas(Shapes, Pid),
  
  
negative.code: 
      {ok, stopped, _Error} = stop(Pid).
  
  server_return_its_state_when_stop_test() ->
      Shapes = [{circle, 0.873}, {square, 0.293}],
      ExpectedState = {{ok, 2.4801478674877355}, Shapes},
      {ok, Pid} = start(), 
  