permanent. Once it is set, it does not go away, even
if the class in question really isn't universal anymore.
-=head2 mro::get_global_sub_generation()
-
-Returns the current value of the internal perl variable
-C<PL_sub_generation>.
-
=head2 mro::invalidate_all_method_caches()
Increments C<PL_sub_generation>, which invalidates method
caching in all packages.
-=head2 mro::get_sub_generation($classname)
-
-Returns the current value of a given package's C<sub_generation>.
-This is only incremented when necessary for that package.
-
-If one is trying to determine whether significant (method/cache-affecting)
-changes have occured for a given stash since you last checked, you should
-check both this and the global one above.
-
=head2 mro::method_changed_in($classname)
Invalidates the method cache of any classes dependent on the