input: plaintext
  Write a function to count the number of vowels in a given string. If the string is empty, return 0.
  def count_vowels(string):
      count = 0
      vowels = set("aeiouAEIOU")
      for letter in string:
          if letter in vowels:
              count += 1
      return count
  
  assert count_vowels("hello")==2
  assert count_vowels("world")==1
  assert count_vowels("aeiou")==5
  assert count_vowels("HELLO")==2
  assert count_vowels("")==0
  
output:     
      count = 0
      vowels = set("aeiouAEIOU")
      for letter in string:
          if letter in vowels:
              count += 1
      return count
  