public function it_can_calculate_unique_houses_delivered_to_from_both_santas(Santa $santa, Santa $robosanta)
 {
     $santa->getHousesDeliveredTo()->willReturn([[0, 0], [1, 0], [1, 1]]);
     $robosanta->getHousesDeliveredTo()->willReturn([[0, 0], [0, 1], [1, 1]]);
     $this->getNumberOfUniqueHousesDeliveredTo()->shouldBe(4);
 }