input: Write a function to find common elements between two sets.
  assert common_elements({1, 2, 3}, {3, 4, 5}) == {3}
  assert common_elements({10, 20, 30}, {30, 40, 50}) == {30}
  assert common_elements({'a', 'b', 'c'}, {'c', 'd', 'e'}) == {'c'}
output: def common_elements(set1, set2):
      return set1 & set2