I'm concerned about merging:

```xml
<d:fileset xml:base="file:/tmp/">
    <d:file href="doc.xml"/>
</d:fileset>
<d:fileset xml:base="file:/tmp/">
    <d:file href="doc.xml" original-base="doc.html"/>
</d:fileset>
<d:fileset xml:base="file:/tmp/">
    <d:file href="doc.xml" original-base="doc.html"/>
</d:fileset>
```

how would the stylesheet react to copying the @original-base attribute twice ? (I don't find the answer off the top of my head, I'd need to test it)