func subTreeIndex(lo, hi int64, need []int64) []int64 {
	// See subTreeHash below for commentary.
	for lo < hi {
		k, level := maxpow2(hi - lo + 1)
		if lo&(k-1) != 0 {
			panic("tlog: bad math in subTreeIndex")
		}
		need = append(need, StoredHashIndex(level, lo>>uint(level)))
		lo += k
	}
	return need
}

// subTreeHash computes the hash for the subtree containing records [lo, hi),