> You are absolutely right when speaking about SQL standard,
> unfortunately Firebird implements Check constraints in a non standard way,
> such that both FALSE and NULL are rejected.

Thanks for the response! I was confused because Firebird was acting that
way I thought it should. Now that I realize it's just an ideosyncracy of
Firebird, it's no big deal. I was afraid that I was totally confused
about how CHECK constraints worked.

Be seeing you.