Does psql use nested transactions?

In 8.0 beta 1 I just tried:
    psql template1    > begin;    > select * from pg_class;    ... got stuff ...    > select * from aaa;    ERROR:
relation"zzz" does not exist    > select * from pg_class;    ERROR: current transaction is aborted....
 

Should psql run every statement in a nested transaction and only rollback 
that TX? Or is that not possible/desirable. If possible, this would be a 
*great* feature for those  of use prone to mis-spellings.



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