Change instanceName

Hi, I try to change the instanceName of my form to the text of the selectfield with autocomplete.
So far I have folowing binding in my xml (build with odk build and changed in my editor):

<bind calculate="concat(/data/art,' - ',/data/datum)" nodeset="/data/meta/instanceName" type="string"/>

This works, but I get the value of my select widget (/data/art) , not the text of the select field.
How can I change this?
Thanks Sven (very new with odk)