A general pattern of a class hierarchy with virtual methods

(define (class-name parameters)
 (let ((super (new-part super-class-name some-parameters))
       (self 'nil))
   (let ((instance-variable init-value)
         ...)
       
     (define (method parameter-list)
       method-body)
     ...

     (define (set-self! object-part)
         (set! self object-part)
         (send 'set-self! super object-part))
       
     (define (dispatch message)
       (cond ((eqv? message 'selector) method)
             ...
             ((eqv? message 'set-self) set-self!)
             (else (method-lookup super message))))
       
     (set! self dispatch))
     
   self))