anchor.diff:          ]).
   
   -export([sync_subscribe/1
  +         ,sync_subscribe/2
            ,sync_unsubscribe/1
           ]).
   
  
positive.code: sync_subscribe(Publisher) ->
      gen_server:call(Publisher, {?MODULE, {sub, self()}}).
  
  sync_unsubscribe(Publisher) ->
      gen_server:call(Publisher, {?MODULE, {unsub, self()}}).
  
  
negative.code: 
  process_msg({sub, Pid}, P = #pubsub{}) ->
      add_subscriber(Pid, P);
  process_msg({sub, Pid, Filter}, P = #pubsub{}) ->
      add_subscriber(Pid, Filter, P);
  process_msg({unsub, Pid}, P = #pubsub{}) ->
      remove_subscriber(Pid, P);
  process_msg({'DOWN', _, _, Pid, _}, P) ->
  