input: Write a python function to find the difference between two sets.
  assert set_difference({1, 2, 3, 4, 5}, {3, 4, 5, 6, 7}) == {1, 2}
  assert set_difference({10, 20, 30}, {20, 30, 40}) == {10}
  assert set_difference({1, 2, 3}, {4, 5, 6}) == {1, 2, 3}
output: def set_difference(set1, set2):
      return set1 - set2