docs: @Override
    public <T> Streamlet<T> flatMap(
        SerializableFunction<R, ? extends Iterable<? extends T>> flatMapFn) {
      checkNotNull(flatMapFn, "flatMapFn cannot be null");
  
      FlatMapStreamlet<R, T> retval = new FlatMapStreamlet<>(this, flatMapFn);
      addChild(retval);
      return retval;
    }