input: def remove_vowels(string):
      """ Remove all vowels from a given string.
      >>> remove_vowels('hello world')
      'hll wrld'
      >>> remove_vowels('this is a test')
      'ths s  tst'
      """
output:     vowels = "aeiouAEIOU"
      return ''.join([char for char in string if char not in vowels])