WHEN и UNLESS - условные операторы

(when p form₁ ... form_n)
  ≡ (if p (progn form₁ ... form_n) nil)
(unless p form₁ ... form_n)
  ≡ (if (not p) (progn form₁ ... form_n) nil)

Пример

(when t
  (prin1 1) (prin1 2) (prin1 3))
123
3
(unless t
  (prin1 1) (prin1 2) (prin1 3))
NIL