docs: def _prepdata(self):
          """Adds potentially missing items to the geojson dictionary"""
          
          # if missing, compute and add bbox
          if not self._data.get("bbox"):
              self.update_bbox()
  
          # if missing, set crs to default crs (WGS84), see http://geojson.org/geojson-spec.html
          if not self._data.get("crs"):
              self._data["crs"] = {"type":"name",
                                 "properties":{"name":"urn:ogc:def:crs:OGC:2:84"}}