The appellants say that the KFG companies are impermissibly using the rule positively to assert a foreign cause of action whose very existence depends (under the lex causae) upon the foreign judgment not existing or having been set aside, and by arguing that the local court can (on the ground that it was allegedly obtained by fraud) treat the foreign judgment as not existing or having been set aside, even though that judgment does actually still exist and has not actually been set aside under the relevant foreign law