code: def _check_missing_manifests(self, segids):
      
      manifest_paths = [ self._manifest_path(segid) for segid in segids ]
      with Storage(self.vol.layer_cloudpath, progress=self.vol.progress) as stor:
        exists = stor.files_exist(manifest_paths)
  
      dne = []
      for path, there in exists.items():
        if not there:
          (segid,) = re.search(r'(\d+):0$', path).groups()
          dne.append(segid)
      return dne
docs: Check if there are any missing mesh manifests prior to downloading.