Common Lisp the Language, 2nd Edition
The :default-initargs class option is inherited. The set of defaulted initialization arguments for a class is the union of the sets of initialization arguments specified in the :default-initargs class options of the class and its superclasses. When more than one default initial value form is supplied for a given initialization argument, the default initial value form that is used is the one supplied by the class that is most specific according to the class precedence list.
If a given :default-initargs class option specifies an
initialization argument of the same name more than once, an
error is signaled.