How to better avoid "Error: free joint can only be used on top level" when combining two models

I want to create separate xml files for robot, object, arena and combine them together later.
Because all the objects are very similar, I decided to put all of them in one xml file. Each object are should move freely.
Something like this:
```XML

```

Then I use .attach to combine two models: modelArena.attach(modelObject)

But, this will create an outside

```
Then I will get a problem "Error: free joint can only be used on top level".

I know one solution would be to create xml file for each object individually and let the attachment frame add('freejoint').
But this is not very elegant, and it will create many redundant attributes (like

先从 modelArena.attach(modelObject) 的用法和 issue 中的 XML 示例开始，然后检查 attachment API 以及它对 worldbody 子项和 freejoint 元素的处理方式。完成的标准是找出并记录一种受支持的方法，用于组合多个可独立移动的 body，同时不会触发顶层 free-joint 错误。

由索引模型根据 Issue 内容生成。