docs: def add_unique_id(self):
          """
          Adds a unique id property to each feature.
  
          Raises:
          
          - An Exception if any of the features already
              have an "id" field. 
          """
          
          uid = 0
          for feature in self._data["features"]:
              if feature["properties"].get("id"):
                  raise Exception("one of the features already had an id field")
              feature["properties"]["id"] = uid
              uid += 1