code: func StatBlob(ctx context.Context, bs BlobStatter, br blob.Ref) (blob.SizedRef, error) {
  	var ret blob.SizedRef
  	err := bs.StatBlobs(ctx, []blob.Ref{br}, func(sb blob.SizedRef) error {
  		ret = sb
  		return nil
  	})
  	if err == nil && !ret.Ref.Valid() {
  		err = os.ErrNotExist
  	}
  	return ret, err
  }
docs: // StatBlob calls bs.StatBlobs to stat a single blob.
  // If the blob is not found, the error is os.ErrNotExist.