How to bind result to one of the object arguments in a TupleFunction?

[michalpie]

Hi,

I need to implement a magic property that would have API similar to the list:index ARQ function (https://jena.apache.org/documentation/query/library-propfunc.html):

list list:index (index member)

Description of the list:index function:
Index of an RDF List (RDF Collection). If list is not bound or a constant, find and iterate all lists in the graph (can be slow) else evaluate for one particular list. The object is a list pair, either element can be bound, unbound or a fixed node. Unbound variables in the object list are bound by the property function.

In my case, I also want to have a mix of bound and unbound arguments in the object list. Is it possible in rdf4j to have e.g. 'index' variable bound and 'member' variable unbound, and make the magic property bind the 'member' property? How to do it (how to properly return result so that it binds the 'member' variable)?
I can see that the list:index is only partially implemented in rdf4j-spin module...

Thanks in advance,
Michal