code: def _add_referencer(registry):
      
      referencer = registry.queryUtility(IReferencer)
      if referencer is not None:
          return referencer
      ref = registry.settings['urireferencer.referencer']
      url = registry.settings['urireferencer.registry_url']
      r = DottedNameResolver()
      registry.registerUtility(r.resolve(ref)(url), IReferencer)
      return registry.queryUtility(IReferencer)
docs: Gets the Referencer from config and adds it to the registry.