input: Write a python function to determine the number of vowels present in a given string.
  assert count_vowels('corner','AaEeIiOoUu') == 2
  assert count_vowels('valid','AaEeIiOoUu') == 2
  assert count_vowels('true','AaEeIiOoUu') == 2
output: def count_vowels(string, vowels):
      return len([char for char in string if char in vowels])