docs: def all_attributes(self):
          """
          Collect and return a list of all attributes/properties/fields used in any of the features.
          """
          features = self._data["features"]
          if not features: return []
          elif len(features) == 1: return features[0]["properties"].keys()
          else:
              fields = set(features[0]["properties"].keys())
              for feature in features[1:]:
                  fields.update(feature["properties"].keys())
              return list(fields)