input: 
  def count_vowels(string):
      """Count the number of vowels in a given string.
      >>> count_vowels("hello")
      2
      >>> count_vowels("world")
      1
      >>> count_vowels("aeiouAEIOU")
      10
      """
      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