input: Write a function that takes in a string and a list of words, and returns true if the string contains all the words in the list, regardless of case. The function should be case insensitive.
  
  def contains_words(input_string, words):
   input_string = input_string.lower()
   for word in words:
   if word.lower() not in input_string:
   return False
   return True
  
  assert contains_words('This is a test', ['test', 'this', 'is'])==True
  assert contains_words('This is a test', ['Test', 'This', 'IS'])==True
  assert contains_words('This is a test', ['test', 'This', 'iss'])==False
  assert contains_words('This is a test', ['test', 'This', 'is'])==True
  assert contains_words('This is a test', [])==True
output: def contains_words(input_string, words):
   input_string = input_string.lower()
   for word in words:
   if word.lower() not in input_string:
   return False
   return True