docs: def Reference(uri, meaning=None):
      """
      Represents external information, typically original obs data and metadata.
  
      Args:
          uri(str): Uniform resource identifier for external data, e.g. FITS file.
          meaning(str): The nature of the document referenced, e.g. what
              instrument and filter was used to create the data?
      """
      attrib = {'uri': uri}
      if meaning is not None:
          attrib['meaning'] = meaning
      return objectify.Element('Reference', attrib)