echo
----
···db0.Txn(ctx, func(ctx context.Context, txn *kv.Txn) error {
···  txn.SetIsoLevel(isolation.Serializable)
···  txn.CreateSavepoint(ctx, 0)
···  txn.Put(ctx, tk(9), sv(3))
···  txn.ReleaseSavepoint(ctx, 0)
···  txn.Get(ctx, tk(8))
···  txn.CreateSavepoint(ctx, 4)
···  txn.Del(ctx, tk(9) /* @s1 */)
···  txn.RollbackSavepoint(ctx, 4)
···  return nil
···})
