input: 
  def count_vowels(string: str) -> int:
      """ Count the number of vowels in a given string.
      >>> count_vowels('Hello World')
      3
      >>> count_vowels('Python Programming')
      4
      """
      count = 0
      vowels = set("aeiouAEIOU")
      for letter in string:
          if letter in vowels:
              count += 1
      return count
  
output:     count = 0
      vowels = set("aeiouAEIOU")
      for letter in string:
          if letter in vowels:
              count += 1
      return count