@Override
  public @NotNull RandomAccessIntContainer ensureContainerCapacity(int diff) {
    int expectedSize = size() + diff;
    if (expectedSize >= myThreshold) {
      upgrade();
    }
    if (myInstanceHigh != null) {
      return myInstanceHigh.ensureContainerCapacity(diff);
    } else {
      return myInstanceLow.ensureContainerCapacity(diff);
    }
  }
@Override
  public @NotNull RandomAccessIntContainer ensureContainerCapacity(int diff)