input: Write a function to find the union of two lists. That means combining two lists into one, but only including each element once, regardless of how many times it appears in both lists.
  
  def unionList(list1, list2):
      return set(list1 + list2)
    
  assert unionList([1, 2, 3, 4, 5], [3, 4, 5, 6, 7])=={1, 2, 3, 4, 5, 6, 7}
  assert unionList([], [3, 4, 5, 6, 7])=={3, 4, 5, 6, 7}
  assert unionList([1, 2, 3], [])=={1, 2, 3}
  assert unionList([1, 2, 3], [4, 5, 6])=={1, 2, 3, 4, 5, 6}
  assert unionList([], [])==set()
output: def unionList(list1, list2):
      return set(list1 + list2)