def test_skew_sum(self):
p1 = Permutation(1243)
p2 = Permutation(15324)
p3 = Permutation(312)
p4 = Permutation(1)
p5 = Permutation()
# All together
result = p1.skew_sum(p2, p3, p4, p5)
expected = Permutation((10,11,13,12,5,9,7,6,8,4,2,3,1))
self.assertEqual(result, expected)
# Two
result = p1.skew_sum(p3)
expected = Permutation((4,5,7,6,3,1,2))
self.assertEqual(result, expected)
# None
self.assertEqual(p1.skew_sum(), p1)
