state.add(ref, null);
state.add(ref);
   public void testRecoverableState_remove_10_mixed() throws Throwable   {      if (!state.isRecoverable())      {         // only recoverable state is tested now         return;      }      for(int i = 0; i < NUMBER_OF_MESSAGES; i++)      {         MessageReference ref =               ms.reference(MessageFactory.createMessage("message" + i, i % 2 == 0, "payload" + i));         state.add(ref, null);      }      for(int i = 0; i < NUMBER_OF_MESSAGES; i++)      {         MessageReference ref = state.remove();         assertEquals("message" + i, ref.getMessageID());      }      assertNull(state.remove());      log.info("ok");   }