pairwise maybe

Observable.pairwise<T>(input : Observable<T> {
    override fun call(subscriber: Subscriber<in T>) {
        let prev, isInitialised = false;
        
        input.subscribe(x -> {
            if (isInitialised)
              subscriber.onNext(new array[prev, x]);
              prev = x;
            } else {
              isInitialised = true;
              prev = curr;
            }
          },
          {
            subscriber.onCompleted()
          }
       );
    }
})