RxSwift extension for flatmap like Swift2.x

optional+swift2LikeFlatmap.swift

extension Observable {
    /**
    example:
    Observable<String>.of("1", "a", "3", "4")
            .flatMap { Int($0) }
            .subscribeNext { print("\($0)") }
    result:
    1
    3
    4
    */
    func flatMap<T>(selector: (E) throws -> T?) -> Observable<T> {
        return flatMap { try selector($0).map { Observable<T>.just($0) } ?? Observable<T>.empty() }
    }
}

extension Driver {
    /**
    example:
    Driver<String>.of("1", "a", "3", "4")
            .flatMap { Int($0) }
            .driveNext { print("\($0)") }
    result:
    1
    3
    4
    */
    func flatMap<T>(selector: (E) -> T?) -> Driver<T> {
        return flatMap { selector($0).map { Driver<T>.just($0) } ?? Driver<T>.empty() }
    }
}