import unittest


def next_bigger(l):
    """
    This function returns a modified list
    by appending a new element that is
    bigger than the max element of the list

    l = [], returns l[0]
    l = [1,2,3], returns [1,2,3,4]
    l = [6,5,9], returns [6,5,9,10]
    """
    if len(l) == 0:
        l.append(0)
    else:
        max_element = max(l)
        l.append(max_element+1)
    return l


class TestNextBigger(unittest.TestCase):

    def test_empty(self):
        self.assertEqual(next_bigger([]), [0])

    def test_negative(self):
        self.assertEqual(next_bigger([-2, -4, -1]), [-2, -4, -1, 0])

    def test_positive(self):
        self.assertEqual(next_bigger([4, 7, 9, 11, 31, 55, 33, 22]),
                        [4, 7, 9, 11, 31, 55, 33, 22, 56])

    def test_mixed(self):
        self.assertEqual(next_bigger([-2, 53, 0, -3, -99]),
                        [-2, 53, 0, -3, -99, 54])

if __name__ == '__main__':
    unittest.main()