Preventing variables from "rolling over" - Mailing list pgsql-general

I assume that if I keep on adding to an INT4, at some point it will
roll over and become negative, and then if I add some more, it could
become positive.  Is there a way to prevent this?  Ie, I would like
for it to end with an error instead of wrapping around.  Or is there a
way to check for this condition before I do the addition?  Is there a
way to put a constraint on a variable, such that it's always positive?

Thanks