public String toString() {
    StringBuilder sb = new StringBuilder("[");
    Node node = _head.next;
    String sep = "";
    while (node != null) {
      sb.append(sep).append("(").append(node.waketime).append(",")
        .append(node.agent).append(")");
      node = node.next;
      sep = ";";
    }
    sb.append("]");
    return (sb.toString());
  }