@Override
  public String toString() {
    return MoreObjects.toStringHelper(this)
        .add("id", id)
        .add("created", created)
        .add("modified", modified)
        .add("name", name)
        .add("source", source)
        .add("location_id", location_id)
        .add("url", url)
        .add("tags", tags)
        .add("participants", participants)
        .add("notes", notes)
        .add("rrule", rrule)
        .add("tasks", tasks)
        .toString();
  }
A tasks series, see: https://www.rememberthemilk.com/services/api/tasks.rtm
extensions/data-transfer/portability-data-transfer-rememberthemilk/src/main/java/org/datatransferproject/transfer/rememberthemilk/model/tasks/TaskSeries.java