code: def exists(self, bbox_or_slices):
      
      if type(bbox_or_slices) is Bbox:
        requested_bbox = bbox_or_slices
      else:
        (requested_bbox, _, _) = self.__interpret_slices(bbox_or_slices)
      realized_bbox = self.__realized_bbox(requested_bbox)
      cloudpaths = txrx.chunknames(realized_bbox, self.bounds, self.key, self.underlying)
      cloudpaths = list(cloudpaths)
  
      with Storage(self.layer_cloudpath, progress=self.progress) as storage:
        existence_report = storage.files_exist(cloudpaths)
      return existence_report
docs: Produce a summary of whether all the requested chunks exist.
  
      bbox_or_slices: accepts either a Bbox or a tuple of slices representing
        the requested volume. 
      Returns: { chunk_file_name: boolean, ... }