old_contents: URL_PATH_BOOK = "/book"
  URL_NAME_BOOK = "book"
  URL_PATH_BUY = "/buy"
  URL_NAME_BUY = "buy"
  URL_PATH_SELL = "/sell"
  URL_NAME_SELL = "sell"
  HTTP_201_CREATED = 201
  HTTP_400_BAD_REQUEST = 400
  HTTP_500_INTERNAL_SERVER_ERROR = 500
  
new_contents: """
  Constants here so we don't sprinkle them throughout the code.
  """
  
  URL_PATH_BOOK = "/book"
  URL_NAME_BOOK = "book"
  URL_PATH_BUY = "/buy"
  URL_NAME_BUY = "buy"
  URL_PATH_SELL = "/sell"
  URL_NAME_SELL = "sell"
  HTTP_201_CREATED = 201
  HTTP_400_BAD_REQUEST = 400
  HTTP_500_INTERNAL_SERVER_ERROR = 500
  
  
subject: Add docstring.