public String toVerticalString() {
    List<E> sortedKeys = toSortedList();
    StringBuffer sb = new StringBuffer();
    for (Iterator<E> keyI = sortedKeys.iterator(); keyI.hasNext();) {
      E key = keyI.next();
      sb.append(key);
      sb.append(" : ");
      sb.append(getPriority(key));
      if (keyI.hasNext()) {
        sb.append("\n");
      }
    }
    return sb.toString();
  }