This question is very Saxon-specific. In future, I would suggest using a
Saxon-specific forum, e.g. saxonica.plan.io.


>
> public Object footnote(Object nr, Object html) {  }
>
> I can cast the nr (which is //footnote/@nr) and html (which is
//footnote/*) to SequenceExtent.

You can't rely on them being supplied as instances of SequenceExtent. For
example, if the value is a singleton or an empty sequence (or in future
releases) a different class might be used.

Better to declare the argument types as net.sf.saxon.om.Sequence, and make no
assumptions beyond them being a Sequence.

>
> Now I can process the footnotes.
>
> As result I want to return the processed footnotes, so that they can be
processed by xsl as an result tree fragment.
>
>
Rather depends how you are constructing them. It might be easiest to use a
tree model that's designed for manipulation in Java, e.g. JDOM2, and then
return a JDOM2 DocumentWrapper. If you want to construct a Saxon tree, you can
instantiate a Builder and then feed it startElement() and endElement() events,
but that's not especially convenient. Ultimately, though, you want to return
an instance of the class DocumentInfo.

Michael Kay
Saxonica