Reuse serial numbers?

["A B" <gentosaker(at)gmail(dot)com>]

Hello.
I have a table
create table TBL (a serial, b integer, c integer, unique(b,c));

so I for each combination of b,c get an unique number a.

Now, if the table is filled and I remove some rows, so there is a gap
in the a numbers. Is there a way to reuse the "free" serial numbers?

I don't feel like writing a script to select all numbers and do some
searching for free numbers.