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))