code: def _prompt(letters='yn', default=None):
      
      while True:
          try:
              input_text = sys.stdin.readline().strip()
          except KeyboardInterrupt:
              sys.exit(0)
          if input_text and input_text in letters:
              return input_text
          if default is not None and input_text == '':
              return default
          print('Come again?')
docs: Wait for the user to type a character (and hit Enter).  If the user enters
      one of the characters in `letters`, return that character.  If the user
      hits Enter without entering a character, and `default` is specified,
      returns `default`.  Otherwise, asks the user to enter a character again.