anchor.diff:  % stats_callback is defined in both erlkaf_consumer_callbacks and erlkaf_producer_callbacks
   % -behaviour(erlkaf_producer_callbacks).
   
  -create(BootstrapServer) ->
  +create() ->
  +    create(undefined).
  +
  +create(BootstrapServers) ->
       erlkaf:start(),
   
       % create consumer
   
       GroupId = atom_to_binary(?MODULE, latin1),
  -    ConsumerConfig = [{bootstrap_servers, BootstrapServer}],
  +    ConsumerConfig = append_bootstrap(BootstrapServers, []),
       TopicConf = [{auto_offset_reset, smallest}],
   
       ok = erlkaf:create_consumer_group(client_consumer, GroupId, [?TOPIC], ConsumerConfig, TopicConf, ?MODULE, []),
   
       % create producer
   
  -    ProducerConfig = [
  -        {bootstrap_servers, BootstrapServer},
  +    ProducerConfig = append_bootstrap(BootstrapServers, [
           {delivery_report_only_error, true},
           {delivery_report_callback, ?MODULE}
  -    ],
  +    ]),
       ok = erlkaf:create_producer(?PRODUCER_CLIENT, ProducerConfig),
       ok = erlkaf:create_topic(?PRODUCER_CLIENT, ?TOPIC, [{request_required_acks, 1}]),
       ok.
  
positive.code: 
      % api
  
      create/1,
      produce/2,
      produce/3,
  
negative.code: 
  stats_callback(ClientId, Stats) ->
      io:format("### producer -> stats_callback: ~p stats:~p ~n", [ClientId, length(Stats)]).
  
  % internals
  
  append_bootstrap(undefined, V) ->
      V;
  append_bootstrap(BootstrapServer, V) ->
      [{bootstrap_servers, BootstrapServer} | V].
  