Different error messages executing CREATE TABLE or ALTER TABLE to create a column "xmin"

Hi all,

trying to create a table with a column xmin I get the
following error message:

test=> create table lx (xmin int);
ERROR: column name "xmin" conflicts with a system
column name

Instead I get a different (and less understandable) error
message if I try to add a column named xmin to an
existent table:

test=> create table lx (i int);
CREATE TABLE
test=> alter table lx add xmin int;
ERROR: column "xmin" of relation "lx" already exists.

The same problem occurs using "xmax" as column name.

I'm on Ubuntu 11.04.
Tried on both PostgreSQL 8.4.10 and 9.1.2

Regards.

--
Giuseppe Sucameli