ANSI Common Lisp 4 Types and Classes 4.3 Classes 4.3.4 Inheritance
188.8.131.52 Inheritance of Class Options
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 supplied 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 of type program-error is signaled.