// tileForIndex returns the tile of height h ≥ 1
// storing the given hash index, which can be
// reconstructed using tileHash(data[start:end]).
func tileForIndex(h int, index int64) (t Tile, start, end int) {
	level, n := SplitStoredHashIndex(index)
	t.H = h
	t.L = level / h
	level -= t.L * h // now level within tile
	t.N = n << uint(level) >> uint(t.H)
	n -= t.N << uint(t.H) >> uint(level) // now n within tile at level