Best approach for a "gap-less" sequence

Hi!

I was trying to solve a problem on an old system and realized that there might
be some better approach for doing what I need.

We have some documents that need to be ordered sequentially and without gaps.
I could use a sequence, but if the transaction fails then when I rollback the
sequence will already have been incremented.

So, today I have a control table and I acquire a SHARE ROW EXCLUSIVE lock to
it, read the value, increase it, do what I need and then I COMMIT the
transaction, ensuring that the sequence has no gaps.

Is there a better way to guarantee that there will be no gaps in my sequence
if something goes wrong with my transaction?

--
Jorge Godoy <jgodoy(at)gmail(dot)com>