关键字参数

可以使用 &KEY 关键字定义关键字参数。它们始终是可选的（有关定义的详细信息，请参阅可选参数示例）。可能有多个关键字参数。

(defun foobar (x y &key (z "Default" zp))
  (format t "X (~s) and Y (~s) are required.~@
             Z (~s) is a keyword argument. It ~:[wasn't~;was~] given.~%"
          x y z zp))

(foobar 10 20)
; X (10) and Y (20) are required.
; Z ("Default") is a keyword argument. It wasn't given.
;=> NIL
(foobar 10 20 :z 30)
; X (10) and Y (20) are required.
; Z (30) is a keyword argument. It was given.
;=> NIL