'Test the variables method.'

def test_variables(self):

    p = Problem(Minimize(self.a), [(self.a <= self.x), (self.b <= (self.A + 2))])
    vars_ = p.variables()
    ref = [self.a, self.x, self.b, self.A]
    if PY2:
        self.assertItemsEqual(vars_, ref)
    else:
        self.assertCountEqual(vars_, ref)
