def testDeletion(self):
slist = SinglyLinkedList()
tail = slist.insert_after(None, 13)
slist.insert_after(None, 12)
actual = to_array(slist)
expected = [12, 13]
self.assertEqual(expected, actual)
slist.delete_after(tail)
actual = to_array(slist)
expected = [12, 13]
self.assertEqual(expected, actual)
slist.delete_after(slist.head)
actual = to_array(slist)
expected = [12]
self.assertEqual(expected, actual)
