inverted optional

inv-opt

(defun test(&rest ab)
            (let ((len (length ab))
                  (a 2) b)
              (cond ((< 2 len) (error "Too many args"))
                    ((= 2 len)
                     (setf a (first ab)
                           b (second ab)))
                    ((= 1 len) (setf b (first ab)))
                    (t (error "B arg is mandatory")))
              (format t "a: ~W, b: ~W~%" a b)))