complement

(complement function)

Returns a predicate answering the opposite of function: the result is t where function returns nil and vice versa. The returned predicate takes any number of arguments -- calls of up to three go through a funcall dispatch, a fourth and beyond through apply, so spelling complement pulls in the apply runtime on the compiled backends. It serves an equality designator (:test / :test-not) as well as a one-argument predicate. Lite: complement expands inline so #'complement is not available.