input: Write a python function to create a bag of words from a given corpus.
  assert create_bag_of_words("the cat in the hat") == {'the', 'in', 'cat', 'hat'}
  assert create_bag_of_words("Python is great and Python is fun") == {'python', 'is', 'great', 'and', 'fun'}
  assert create_bag_of_words("Hello world! Hello universe.") == {'hello', 'world!', 'universe.'}
output: def create_bag_of_words(corpus):
      words = corpus.lower().split()
      return set(words)