(defvar *invoices* (ds:make-dictionary))

;;; WAY 1
(defgeneric make-inst (class &rest initargs &key customize &allow-other-keys))
(defmethod make-inst (class &rest initargs &key (customize t) &allow-other-keys)
  (apply #'make-instance class :customize customize initargs))

(defmethod initialize-instance :around ((inv invoice:invoice) &rest initargs &key customize &allow-other-keys)
  (let ((instance (call-next-method)))
    (when customize
      (ds:insert instance *invoices*))
    instance))

(make-instance 'invoice:standard-invoice)
(make-inst 'invoice:standard-invoice) ; creates an instance and also populates *invoices*

;;; WAY 2 which I think is not as good
(defgeneric make-invoice (class &rest initargs &key &allow-other-keys))
(defgeneric make-inst (class &rest initargs &key &allow-other-keys))

(defmethod make-inst (class &rest initargs &key &allow-other-keys)
  (apply (cond ((subtypep class 'invoice:invoice)
		#'make-invoice)
	       (t
		#'make-instance))
	 class initargs))

(defmethod make-invoice :around (class &rest initargs &key &allow-other-keys)
  (let ((instance (call-next-method)))
    (ds:insert instance *invoices*)))