// rollover, causing an immediate expiration (ironic, eh?)
const forever time.Duration = 1<<(63-1) - 1

// newSMT creates a new smt given a keySize, hash function, cache (nil will be defaulted to TTLCache), and retention
// duration for old nodes.
func newSMT(hash func(data ...[]byte) []byte, updateCache cache.ExpiringCache, retentionDuration time.Duration) *smt {
	if updateCache == nil {