def test(self):
   self.assertEqual(gcd_recur(0, 2), 2)
 def test(self):
   self.assertEqual(gcd_recur(4, 0), 4)
 def test(self):
   self.assertEqual(gcd_recur(49, -21), 7)
 def test(self):
   self.assertEqual(gcd_recur(5, 13), 1)