Why does flatMap interface return RStream<T> instead of RStream<VR> ？

[huangql]

RStream flatMap(final ValueMapperAction<T, ? extends Iterable<? extends VR>> mapper);

Why does this interface return RStream instead of RStream ？
How do I get RStream ？
I want to keyBy with VR.id

[ni-ze]

@huangql It does a bug, can you submit a pr to correct it?