Depends on the language/type safety offered. If your language has a NaN (javascript) and you can assert the expected behavior of x < NaN (false) or if you don't even want to encounter that, then you're able to reason back to the generation of the inputs. Wherever x and y came from, if there was a possibility of an NaN being a value, you want the generating function to exception or cast via some sort of validation before handing them off to other unsuspecting functions.