*/
private void compressDownward(long seedNode) {
double threshold = Math.floor(size / compressionFactor);
// P2 check same as above but shorter and slower (and invoked rarely)
LongArrayFIFOQueue q = new LongArrayFIFOQueue();
q.enqueue(seedNode);
while (!q.isEmpty()) {
long node = q.dequeueLong();
long atNode = get(node);
long atSibling = get(sibling(node));
if (atNode == 0 && atSibling == 0) {