RPL: "not" operator?

Wondering whether RPL offers an a "not" or "inverse" operator to reverse the meaning of a function—particularly for exp?isnull

I'm trying to say, "If neither X nor Y is null, then [do this]"

...but since "is not null" doesn't seem to exist, should I simply tackle this using exp?length instead? ("if both expressions have length greater than 0, then [do this]")

Thanks for any help!!