code: func (ch *CRLChecker) IsRevokedSN(c context.Context, sn *big.Int) (bool, error) {
  	snBlob, err := utils.SerializeSN(sn)
  	if err != nil {
  		return false, err
  	}
  	shard, err := ch.shard(c, shards.ShardIndex(snBlob, ch.shardCount))
  	if err != nil {
  		return false, err
  	}
  	_, revoked := shard[string(snBlob)]
  	return revoked, nil
  }
docs: // IsRevokedSN returns true if given serial number is in the CRL.