public List<? extends PackagingElement<?>> addOrFindChildren(Collection<? extends PackagingElement<?>> children) {
    List<PackagingElement<?>> added = new ArrayList<>();
    for (PackagingElement<?> child : children) {
      added.add(addOrFindChild(child));
    }
    return added;
  }
public List<? extends PackagingElement<?>> addOrFindChildren(Collection<? extends PackagingElement<?>> children)