Subselects and NOTs

Oracle 6, there is NULL into table b:

SQL> select * from a where x in (select * from b);

         X
----------
         2

SQL> select * from a where x not in (select * from b);

no rows selected

SQL> select * from a where not x in (select * from b);

no rows selected

Is 'not X in' the same as 'X not in' ? Currently we have:

vac=> select * from a where not x in (select * from b);
x
-
1
(1 row)

: subselect clause is "atomic" and NOT-s are never pushed into it.

Once again - what standard says ?

Vadim