input: 
  def replace_all_occurrences(str, char, replacement):
      """ Replace all occurrences of the character `char` in the string `str` with `replacement`.
      >>> replace_all_occurrences("Hello World!", 'l', 'X')
      'HeXXo WorXd!'
      >>> replace_all_occurrences("Hello World!", 'o', 'Y')
      'HellY WYrld!'
      """
      return str.replace(char, replacement)
  
output:     return str.replace(char, replacement)