²This definition is required when multiple inheritance comes into play. Then, two classes have to be compared with respect to a third class even if they are not related to each other via the subclass relationship. Although, multiple inheritance is not provided at level-0, it is provided at level-1 the method lookup protocol is independent of the inheritance strategy defined on classes. It depends on the class precedence lists of the domains of methods attached to the generic function and the argument classes involved.