Create test cases for the given code

use the given outline code

from __future__ import annotations

import unittest

from perm_lex import perm_gen_lex

class Tests(unittest.TestCase):

def test_perm_gen_lex_empty(self):

self.assertEqual(perm_gen_lex(""), [""])

# TODO: add more tests

if __name__ == "__main__":

unittest.main()