+
+(defmethod invalid-qualifiers ((gf generic-function)
+ combin
+ method)
+ (let ((qualifiers (method-qualifiers method)))
+ (let ((why (cond
+ ((cdr qualifiers) "has too many qualifiers")
+ (t (aver (not (member (car qualifiers)
+ '(:around :before :after))))
+ "has an invalid qualifier"))))
+ (invalid-method-error
+ method
+ "The method ~S on ~S ~A.~%~
+ Standard method combination requires all methods to have one~%~
+ of the single qualifiers :AROUND, :BEFORE and :AFTER or to~%~
+ have no qualifier at all."
+ method gf why))))