docs: @Override
    public <T> Streamlet<T> transform(
        SerializableTransformer<R, ? extends T> serializableTransformer) {
      checkNotNull(serializableTransformer, "serializableTransformer cannot be null");
  
      TransformStreamlet<R, T> transformStreamlet =
          new TransformStreamlet<>(this, serializableTransformer);
      addChild(transformStreamlet);
      return transformStreamlet;
    }