code: func (mb *referenceManifestBuilder) AppendReference(d distribution.Describable) error {
  	r, ok := d.(Reference)
  	if !ok {
  		return fmt.Errorf("unable to add non-reference type to v1 builder")
  	}
  
  	// Entries need to be prepended
  	mb.Manifest.FSLayers = append([]FSLayer{{BlobSum: r.Digest}}, mb.Manifest.FSLayers...)
  	mb.Manifest.History = append([]History{r.History}, mb.Manifest.History...)
  	return nil
  
  }
docs: // AppendReference adds a reference to the current ManifestBuilder