docs: private synchronized LinkedHashSet<IMessageCallback> getCallbacks(String channel, boolean autoCreate, boolean clone) {
          LinkedHashSet<IMessageCallback> result = callbacks.get(channel);
          
          if (result == null && autoCreate) {
              callbacks.put(channel, result = new LinkedHashSet<>());
          }
          
          return result == null ? null : clone ? new LinkedHashSet<>(result) : result;
      }