def number_to_name(number):
  """ helper function to translate numbered choices to string names
  >>>number_to_name(0)
  'rock'
  >>>number_to_name(5)
  None
  """
  if (number == 0):
    return 'rock'
  elif (number == 1):
    return 'spock'
  elif (number == 2):
    return 'paper'
  elif (number == 3):
    return 'lizard'
  elif (number == 4):
    return 'scissors'
  else:
    return None