input: Write a function in Python to count the number of vowels in a string.
  
  Input: A string representing the input text.
  
  Output:
  
  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 World")==3
  assert count_vowels("Python Programming")==4
  assert count_vowels("OpenAI")==4
  assert count_vowels("ABCDEFGHIJKLMNOPQRSTUVWXYZ")==5
  assert count_vowels("12345")==0
output: def count_vowels(string):
      count = 0
      vowels = set("aeiouAEIOU")
      for letter in string:
          if letter in vowels:
              count += 1
      return count