Currently, flatMapSingle returns Single and signals NoSuchElementException if the source Maybe is empty. flatMapSingleElement remains Maybe.

I'd consider renaming flatMapSingle to flatMapSingleOrError and flatMapSingleElement to flatMapSingle as I think a default Maybe -> Maybe transformer should better match the other such transformers.

Alternatively, drop flatMapSingle and rename flatMapSingleElement to flatMapSingle so that the original behavior could be achieved via flatMapSingle.toSingle()).