SW-MVC> (defstruct super 
          (x)
          (y))
SUPER
SW-MVC> (defstruct (sub (:include super))
          (z))
SUB
SW-MVC> (defmethod test ((obj super))
          (format t "X: ~A, Y: ~A~%" (super-x obj) (super-y obj)))
#<STANDARD-METHOD TEST (SUPER) {ADFEC41}>
SW-MVC> (defmethod test ((obj sub))
          (format t "X: ~A, Y: ~A, Z: ~A~%" 
                  (super-x obj) (super-y obj) ;; SUB-X and SUB-Y would also work here.
                  (sub-z obj)))                   ;; But here we must use SUB-Z.
#<STANDARD-METHOD TEST (SUB) {BF71C01}>
SW-MVC> (test (make-super :x 1 :y 2))
X: 1, Y: 2
NIL
SW-MVC> (test (make-sub :x 1 :y 2 :z 3))
X: 1, Y: 2, Z: 3
NIL
SW-MVC>