echo
----
db0.Txn(ctx, func(ctx context.Context, txn *kv.Txn) error {
  txn.SetIsoLevel(isolation.Serializable)
  txn.Put(ctx, tk(1), sv(1)) // <nil>
  txn.Put(ctx, tk(2), sv(2)) // <nil>
  return nil
}) // result is ambiguous: boom
/Table/100/"0000000000000001"/0.000000001,0 @ s1 v1
/Table/100/"0000000000000002"/0.000000002,0 @ s2 v2
ambiguous serializable txn non-atomic timestamps: [w]/Table/100/"0000000000000001":0.000000001,0->v1@s1 [w]/Table/100/"0000000000000002":0.000000002,0->v2@s2
