The ':isa' message selector tests if an object inherits from a class.
(setq a-class (send class :new '(state)))   ; create a new class A-CLASS with STATE
(send a-class :answer :isnew '()            ; set up initialization
                             '((setq state nil) self))
(send a-class :answer :set-it '(value)      ; create :SET-IT message
                              '((setq state value)))
(setq an-obj (send a-class :new))           ; create AN-OBJ out of A-CLASS
(send an-obj :show)                         ; returns object - STATE = NIL
(send an-obj :set-it 5)                     ; STATE is set to 5
(send an-obj :show)                         ; returns object - STATE = 5
(SEND an-obj :ISNEW)                        ; re-initialize AN-OBJ
(send an-obj :show)                         ; returns object - STATE = NIL