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