The way it currently works:

```xml
<d:fileset xml:base="file:/A/">
    <d:file href="file1" original-href="file:/absolutely-preserved/"/>
    <d:file xml:base="file:/A/B/" href="file2"/>
</d:fileset>
```

...when given `$new-base='file:/C/'` becomes...

```xml
<d:fileset xml:base="file:/C/">
    <d:file href="file1" original-href="file:/absolutely-preserved/"/>
    <d:file href="B/file2"/>
</d:fileset>
```

If you want to preserve `@xml:base` we'd have to relativize against `$new-base` instead of `base-uri(/*)` indeed.