docs: def send(self, diff):
          """ Write the diff (toVol from fromVol) to the stream context manager. """
          path = self._fullPath(self.extraKeys[diff])
          keyName = self._keyName(diff.toUUID, diff.fromUUID, path)
          key = self.bucket.get_key(keyName)
  
          if self._skipDryRun(logger)("send %s in %s", keyName, self):
              return None
  
          progress = _BotoProgress(diff.size) if self.showProgress is True else None
          return _Downloader(key, progress)