bigint overflow 'feature'

Just wondering: Is this a known 'feature' & is there a workaround (to catch the overflow and abort the txn)??:

www=# select bigint '9223372036854775807' - bigint '-9223372036854775807';
 ?column?
----------
       -2
(1 row)


PostgreSQL 7.1.3 just downloaded.


Allan.