code: public static <T> Transformer<T, T> doOnNext(final int n, final Action1<? super T> action) {
          return new Transformer<T, T>() {
              @Override
              public Observable<T> call(Observable<T> o) {
                  return o.lift(OperatorDoOnNth.create(action, n));
              }
          };
      }
docs: Returns a {@link Transformer} that applied to a source {@link Observable}
  calls the given action on the {@code n}th onNext emission.
  
  @param n
  the 1-based count of onNext to do the action on
  @param action
  is performed on {@code n}th onNext.
  @param <T>
  the generic type of the Observable being transformed
  @return Transformer that applied to a source Observable calls the given
  action on the nth onNext emission.