input: Write a function to take two lists as input and return their union as a set. The union of two lists is a set containing elements that are in either of the lists.
  
  list1 = [1, 2, 3, 4, 5]
  list2 = [3, 4, 5, 6, 7]
  
  def unionList(list1, list2):
      return set(list1 + list2)
  
  assert unionList(list1, list2) == {1, 2, 3, 4, 5, 6, 7}
  assert unionList([], []) == set()
  assert unionList([1, 2, 3], [4, 5, 6]) == {1, 2, 3, 4, 5, 6}
  assert unionList([1, 1, 1], [2, 2, 2]) == {1, 2}
  assert unionList([1, 2, 3], [1, 2, 3]) == {1, 2, 3}
output: def unionList(list1, list2):
      return set(list1 + list2)