input: Write a function to count the total number of vowels in a given string. A vowel is any of the letters 'a', 'e', 'i', 'o', or 'u'. The function should be case-insensitive.
  
  def count_vowels(string): 
      vowels = "aeiouAEIOU"
      count = 0
      for s in string:
          if s in vowels:
              count += 1
      return count
  
  assert count_vowels("Hello World")==3
  assert count_vowels("Python Programming")==4
  assert count_vowels("AEIOUaeiou")==10
  assert count_vowels("")==0
  assert count_vowels("bcdfghjklmnpqrstvwxyz")==0
output: def count_vowels(string): 
      vowels = "aeiouAEIOU"
      count = 0
      for s in string:
          if s in vowels:
              count += 1
      return count