(defpackage #:hello-world
  (:use #:cl)
  (:export #:hello))
(in-package #:hello-world)

(defun hello NIL)

(defpackage #:hello-world
  (:use #:cl)
  (:export #:hello))
(in-package #:hello-world)

(defun hello ())

Replace `NIL` with `()` in stub function definition.
Replace `NIL` with `()` in stub function definition.

While equivalent, definitely not idiomatic.

exercism/xlisp,exercism/xlisp