def test_equation_cancellation_is_applicable(self):
lhs = Parser(Tokenizer.tokenize('x + 4')).parse()
rhs = Parser(Tokenizer.tokenize('y')).parse()
equation = Equation(lhs, rhs)
addition_cancellation = EquationCancellation(OperationType.PLUS(),
OperationType.MINUS())
self.assertTrue(addition_cancellation.is_applicable_to(equation))
flipped = equation.flip()
self.assertFalse(addition_cancellation.is_applicable_to(flipped))
