code: @Override
    public void writeExternal(ObjectOutput out) throws IOException {
      out.writeInt(id);
      out.writeInt(DBIDUtil.asInteger(routingObjectID));
      out.writeDouble(parentDistance);
      out.writeDouble(coveringRadius);
    }
docs: Calls the super method and writes the routingObjectID, the parentDistance
  and the coveringRadius of this entry to the specified stream.