Interface indicating an operator implementation can be macro-fused back to Maybe in case the operator goes from Maybe to some other reactive type and then the sequence calls for toMaybe again:

Single<Integer> single = Maybe.just(1).isEmpty();
Maybe<Integer> maybe = single.toMaybe();

The Single.toMaybe() will check for this interface and call the fuseToMaybe() to return a Maybe which could be the Maybe-specific implementation of isEmpty().

This causes a slight overhead in assembly time (1 instanceof check, 1 operator allocation and 1 dropped operator) but does not incur the conversion overhead at runtime.