Translating constraints to RM Terms

I'm wondering if some RM theorist might express something for me.

Consider table X with columns A and B. There is a constraint that A must be less than B. While we discuss such constraints every day, I realize I do not know how to express such constraints in relational terms.

For instance, A is actually a domain, as is B, but the constraint A < B must in fact be part of the very definition of the domain A, and here we are defining one domain in terms of another. I realized I have not seen this disccussed in the year or so I have been a regular here. Is defining one domain in terms of another allowed and considered trivial, no big deal? Is it actually not allowed and this is some SQL alteration of true RM?

-- 
Kenneth Downs
Secure Data Software, Inc.
(Ken)nneth@(Sec)ure(Dat)a(.com)