FlatMapWithIndex.swift

let killua = Player(level: Variable(50))
let gon = Player(level: Variable(50))
let bisque = Player(level: Variable(70))

let player = PublishSubject<Player>()
player.asObservable()
    .flatMapWithIndex { player, index -> Observable<Int> in
        if index < 2 {
            return player.level.asObservable()
        }
        return .empty()
    }
    .subscribe(onNext: { print($0) })
    .disposed(by: disposeBag)

player.onNext(killua) // 50
killua.level.value = 51 // 51
player.onNext(gon) //50
gon.level.value = 51 // 51
player.onNext(bisque) // level of bisque isn't printed.
killua.level.value = 52 // 52
gon.level.value = 52 // 52