15.2.1 Inheritance across metaclasses

Usually an inherited class is of the same metaclass as the parent class.

For other kinds of inheritance, you need to define a method on validate-superclass which returns true when called with the respective metaclasses. For example:

(defclass mclass-1 (standard-class)

  ())

(defclass mclass-2 (standard-class)

  ())

(defclass a ()

  ()

  (:metaclass mclass-1))

(defmethod validate-superclass 

           ((class mclass-2) 

            (superclass mclass-1)) 

  t)

(defclass b (a)

  ()

  (:metaclass mclass-2))

Without the validate-superclass method, the last form signals an error because mclass-1 is an invalid superclass of mclass-2 .