Currently,flatMapSingle returnsSingle and signalsNoSuchElementException if the sourceMaybe is empty.flatMapSingleElement remainsMaybe.

I'd consider renamingflatMapSingle toflatMapSingleOrError andflatMapSingleElement toflatMapSingle as I think a defaultMaybe ->Maybe transformer should better match the other such transformers.

Alternatively, dropflatMapSingle and renameflatMapSingleElement toflatMapSingle so that the original behavior could be achieved viaflatMapSingle.toSingle()).