> (setq x (make-test :a 1 :b 2 :c 3)) #S(TEST :A 1 :B 2 :C 3) > (setq y (make-test :a 4)) #S(TEST :A 4 :B NIL :C NIL) > (test-a x) 1 > (test-b y) NIL > (test-c x) 3 > (setf (test-b y) 10) 10 > y #S(TEST :A 4 :B 10 :C NIL) > (setf (test-c y) 20) 20 > y #S(TEST :A 4 :B 10 :C 20)