// duration for old nodes.
func newSMT(hash func(data ...[]byte) []byte, updateCache cache.ExpiringCache, retentionDuration time.Duration) *smt {
	if updateCache == nil {
		updateCache = cache.NewTTL(forever, time.Second)
	}
	s := &smt{
		hash:              hash,
		trieHeight:        len(hash([]byte("height"))) * 8, // hash any string to get output length
		retentionDuration: retentionDuration,
	}