Consider a class with a trivial initialization dependency between slots A and B: 

 (defclass super ()
  ((a :initarg :a :reader a)
   (b :initform 0 :initarg :b :reader b)))
 
 (defmethod initialize-instance :after ((super super) &key &allow-other-keys)
  (setf (slot-value super 'a) (1+ (slot-value super 'b))))

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