def test_size(self):
s = Sustitucion()
self.assertEqual(s.size(), 0)
s.push(1, 2)
self.assertEqual(s.size(), 1)
s.pop()
self.assertEqual(s.size(), 0)
def test_pop(self):
s = Sustitucion()
s.push(1, 2)
self.assertEqual(s.pop(), (1, 2))
