self.assertEqual(self.pi.subs('x', 3), self.pi)
self.assertEqual(self.expr.subs('x', -3), -2 * self.y)
self.assertEqual(self.expr.subs([('x', self.y), ('y', self.x)]), 3 - self.x)
self.assertEqual(self.pi.subs('x', 3), self.pi)
self.assertEqual(self.expr.subs('x', -3), -2 * self.y)
self.assertEqual(self.expr.subs([('x', self.y), ('y', self.x)]), 3 - self.x)