void newTransactionTab(int transactionId)
  {
    if (transactionPane.getTabCount() >= MAX_NOF_TABS)
      transactionPane.remove(0);
    OutputArea oa = new OutputArea("Output of transaction " + transactionId + ':');
    transactionPane.addTab(String.valueOf(transactionId), oa);
    transactionPane.setSelectedComponent(oa);
  }