>>444
classの方でやればできるな。
(defun x-to-y (a)
　"Dummy implementation"
　(1+ a))
(defclass hoge ()
　((x :accessor my-hoge-x :initarg :x :initform 0)
　 (y :accessor hoge-y :initarg :y :initform 0)))
(defun hoge-x-setter (h new-x)
　"Exposed writer for the slot X of HOGE"
　(setf (my-hoge-x h) new-x)
　(setf (hoge-y h) (x-to-y new-x)))
(defsetf hoge-x hoge-x-setter)
(defun hoge-x (h)
　"Exposed reader for the slot X of HOGE"
　(my-hoge-x h))