input: Write a function to determine if a given string contains all the words in a list of words. The function should take a string and a list of words as input. If the string contains all the words, return True; otherwise, return False.
  assert contains_words('This is a test', ['test', 'this', 'is'])==True
  assert contains_words('This is a test', ['test', 'that', 'is'])==False
  assert contains_words('Hello world', ['world', 'hello'])==False
  assert contains_words('', ['test'])==False
  assert contains_words('Python is great', ['python', 'is', 'great'])==False
output: def contains_words(s, words):
      s_lower = s.lower()
      return all(word.lower() in s_lower for word in words)