private void assertAllDeltasAreNull(@Nullable IntervalNode<T> node) {
    if (node == null) return;
    if (!node.isValid()) return;
    assert node.delta == 0;
    long packedOffsets = node.cachedDeltaUpToRoot;
    assert IntervalNode.modCount(packedOffsets) != getModCount() || IntervalNode.allDeltasUpAreNull(packedOffsets);
  }
private void assertAllDeltasAreNull(@Nullable IntervalNode<T> node)