input: Write a function to find the union of two lists. The function should return a set containing all unique elements from both input lists.
  def union_of_lists(list1, list2):
      return set(list1 + list2)
  
  assert union_of_lists([1, 2, 3, 4, 5], [3, 4, 5, 6, 7]) == {1, 2, 3, 4, 5, 6, 7}
  assert union_of_lists([], [1, 2]) == {1, 2}
  assert union_of_lists([1, 1, 1], [2, 2, 2]) == {1, 2}
  assert union_of_lists([5, 4, 3], [3, 2, 1]) == {1, 2, 3, 4, 5}
  assert union_of_lists([], []) == set()
output: def union_of_lists(list1, list2):
      return set(list1 + list2)