'Test the parameters method.'

def test_parameters(self):

    p1 = Parameter()
    p2 = Parameter(3, sign='negative')
    p3 = Parameter(4, 4, sign='positive')
    p = Problem(Minimize(p1), [((self.a + p1) <= p2), (self.b <= ((p3 + p3) + 2))])
    params = p.parameters()
    ref = [p1, p2, p3]
    if PY2:
        self.assertItemsEqual(params, ref)
    else:
        self.assertCountEqual(params, ref)
