Halakhah к Рош ха-Шана 4:11

21. We do not break the laws of Holidays for the Rosh Hashanah shofar, even something that is only rabbinically prohibited. How so? If there was a shofar at the top of a tree or across a river and there is no other shofar, one does not climb the tree or sail on the water. Having a non-Jew do these things is permissible, because that is a rabbinic prohibition on top of a rabbinic prohibition.(Meaning, the prohibition of telling a non-Jew to do an action [on a holiday] that is prohibited by the Torah is a rabbinically enacted prohibition to ensure resting [shevut] from telling non-Jews to do things, and when the action in question is itself a rabbinic prohibition, this is a rabbinic prohibition on top of a rabbinic prohibition [shevut d'shevut]) and when there is a commandment to do, the rabbis made no such decree (see 307).