This change breaks backward compatibility, but the fix is trivial (don't
expect any value to be returned), so I'd rather do this change now than
later.
panic() call is a better fit to handle Define() errors, as the only
errors here are the ones of program logic, not something that program
users' can fix providing different input.
The proper solution would be for Define() to panic itself and not return
anything at all, but I'm leaving it as it for the sake of backwards
compatibility as it is mostly a cosmetic issue.
