@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final <U extends Collection<? super T>> Observable<U> buffer(
            long timespan, TimeUnit unit,
            Scheduler scheduler, int count,
            Callable<U> bufferSupplier,
            boolean restartTimerOnMaxSize) {
        ObjectHelper.requireNonNull(unit, "unit is null");
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");
        ObjectHelper.requireNonNull(bufferSupplier, "bufferSupplier is null");
        ObjectHelper.verifyPositive(count, "count");
        return RxJavaPlugins.onAssembly(new ObservableBufferTimed<T, U>(this, timespan, timespan, unit, scheduler, bufferSupplier, count, restartTimerOnMaxSize));
    }