(defstruct s-fixnum :type s-fixnum
  (n 0 :type fixnum)
  (doc "" :type string))

(defclass foo ()
  ((a :accessor a :type 'fixnum :initarg :a)))

(defclass bar (foo)
  ((a :type 's-fixnum :initarg :a)))

(defmethod a ((i bar))
  (s-fixnum-n (slot-value i 'a)))