* Returns a serializable form of this object. Non-public subclasses should not override this
   * method. Publicly-accessible subclasses must override this method and should return a subclass
   * of SerializedForm whose readResolve() method returns objects of the subclass type.
   */
  @J2ktIncompatible // serialization
  Object writeReplace() {
    return new SerializedForm<>(this);
  }