self.assertEqual(self.e.coefficient('x'), 1)
self.assertEqual(self.e.coefficient('y'), -2)
self.assertEqual(self.e.coefficient(self.y), -2)
self.assertEqual(self.e.coefficient('x'), 1)
self.assertEqual(self.e.coefficient('y'), -2)
self.assertEqual(self.e.coefficient(self.y), -2)