func simpleHashFromHashes(hashes [][]byte) []byte {
	// Recursive impl.
	switch len(hashes) {
	case 0:
		return nil
	case 1:
		return hashes[0]
	default:
		left := simpleHashFromHashes(hashes[:(len(hashes)+1)/2])
		right := simpleHashFromHashes(hashes[(len(hashes)+1)/2:])
		return SimpleHashFromTwoHashes(left, right)
	}
}
func simpleHashFromHashes(hashes [][]byte) []byte {
	// Recursive impl.
	switch len(hashes) {
	case 0:
		return nil
	case 1:
		return hashes[0]
	default:
		left := simpleHashFromHashes(hashes[:(len(hashes)+1)/2])
		right := simpleHashFromHashes(hashes[(len(hashes)+1)/2:])
		return SimpleHashFromTwoHashes(left, right)
	}
}
//----------------------------------------------------------------
// Expects hashes!