SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
                                ^^^^ required

Also note that SET TRANSACTION ISOLATION LEVEL change
isolevel for _current_ transaction, in accordance with
standard, not for session (ALTER SESSION is used in Oracle,
but it's not implemented currently).

And I would don't mention SET XACTISOLEVEL TO ...
form at all.

Please update set.sgml - I failed to understand all these

SET TIME ZONE { '<REPLACEABLE CLASS="PARAMETER">

now.

for Vadim