docs: def find_identifier(self):
          """Find a unique identifier for each feature, create it if needed."""
          features = self.data['features']
          n = len(features)
          feature = features[0]
          if 'id' in feature and len(set(feat['id'] for feat in features)) == n:
              return 'feature.id'
          for key in feature.get('properties', []):
              if len(set(feat['properties'][key] for feat in features)) == n:
                  return 'feature.properties.{}'.format(key)
          if self.embed:
              for i, feature in enumerate(self.data['features']):
                  feature['id'] = str(i)
              return 'feature.id'
          raise ValueError(
              'There is no unique identifier for each feature and because '
              '`embed=False` it cannot be added. Consider adding an `id` '
              'field to your geojson data or set `embed=True`. '
          )