code: func (rl *roLayer) TarStreamFrom(parent ChainID) (io.ReadCloser, error) {
  	var parentCacheID string
  	for pl := rl.parent; pl != nil; pl = pl.parent {
  		if pl.chainID == parent {
  			parentCacheID = pl.cacheID
  			break
  		}
  	}
  
  	if parent != ChainID("") && parentCacheID == "" {
  		return nil, fmt.Errorf("layer ID '%s' is not a parent of the specified layer: cannot provide diff to non-parent", parent)
  	}
  	return rl.layerStore.driver.Diff(rl.cacheID, parentCacheID)
  }
docs: // TarStreamFrom does not make any guarantees to the correctness of the produced
  // data. As such it should not be used when the layer content must be verified
  // to be an exact match to the registered layer.