public String toString() {
    StringBuilder sb = new StringBuilder("[");
    String sep = "";
    Node[] nodes = _queue.toArray(new Node[0]);
    java.util.Arrays.sort(nodes);
    for (Node node : nodes) {
      sb.append(sep).append("(").append(node.waketime).append(",")
        .append(node.agent).append(")");
      sep = ";";
    }
    sb.append("]");
    return (sb.toString());
  }