// Convenience extension access when the type has an associated EXTENSION ID:
template <class U> U* extension() const { return extension<U>(U::EXTENSION_ID); }
template<class U> void extension_set(U* p) { extension_set<U>(U::EXTENSION_ID, p); }
};
// Convenience extension access when the type has an associated EXTENSION ID:
template <class U> U* extension() const { return extension<U>(U::EXTENSION_ID); }
template<class U> void extension_set(U* p) { extension_set<U>(U::EXTENSION_ID, p); }
};