code: def oai_identifier_description(scheme, repositoryIdentifier,
                                 delimiter, sampleIdentifier):
      
      oai_identifier = Element(etree.QName(NS_OAI_IDENTIFIER[None],
                               'oai_identifier'),
                               nsmap=NS_OAI_IDENTIFIER)
      oai_identifier.set(etree.QName(ns['xsi'], 'schemaLocation'),
                         '{0} {1}'.format(OAI_IDENTIFIER_SCHEMA_LOCATION,
                                          OAI_IDENTIFIER_SCHEMA_LOCATION_XSD))
      oai_identifier.append(E('scheme', scheme))
      oai_identifier.append(E('repositoryIdentifier', repositoryIdentifier))
      oai_identifier.append(E('delimiter', delimiter))
      oai_identifier.append(E('sampleIdentifier', sampleIdentifier))
      return etree.tostring(oai_identifier, pretty_print=True)
docs: Generate the oai-identifier element for the identify response.
  
      The OAI identifier format is intended to provide persistent resource
      identifiers for items in repositories that implement OAI-PMH.
      For the full specification and schema definition visit:
      http://www.openarchives.org/OAI/2.0/guidelines-oai-identifier.htm