Addition and subtraction on BIT type

Hello,
Is there a way to do addition and subtraction on BIT types?  For example,
for
creat table test (a BIT(3));
insert into test values (B'101');

select a + 1 from test; fails

and select a::smallint + 1 from test; also fails.

In addition, is there a way to change the bit of a bit string?  For
example change a 1 to a 0 or vice versa.

Any suggestions?
Thanks,
Yasir