<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def delete_os_dummy_rtr_nwk(self, rtr_id, net_id, subnet_id):
 """Delete the dummy interface to the router. """

        subnet_lst = set()
        subnet_lst.add(subnet_id)
        ret = self.os_helper.delete_intf_router(None, None, rtr_id, subnet_lst)
        if not ret:
            return ret
        return self.os_helper.delete_network_all_subnets(net_id)