code: func (x *Index) IsDeleted(br blob.Ref) bool {
  	if x.deletes == nil {
  		// We still allow the slow path, in case someone creates
  		// their own Index without a deletes cache.
  		return x.isDeletedNoCache(br)
  	}
  	x.deletes.RLock()
  	defer x.deletes.RUnlock()
  	return x.isDeleted(br)
  }
docs: // IsDeleted reports whether the provided blobref (of a permanode or
  // claim) should be considered deleted.