Same with "Bob's age is not 30". Age is a variable so you have to covert it to a domain. In order to do that you have to explicitly say what "not 30" means. I'd

Same with "Bob's age is not 30". Age is a variable so you have to covert it to a domain. In order to do that you have to explicitly say what "not 30" means. I'd say it means 1 - 29, 31 - 120. But look what happened. The original, variable, version depended on implicit information in people's brains regarding the possible range of human ages. The domain version makes this information explicit. This is exactly what rules/sucks about relational databases. Everything is harder because you can't get away with statements that are subjectively ambiguous.