Detailed search would be more helpful, if the "exact" mode would not only allow inclusion of instruments but also exclusion.

E.g. searching for duetts sopano/bass and arbitrary accompaniment I get the following search results:
- Without "exact" all works for terzet,quartett, etc, so the duetts are very hard to find amidst the plenty of results.
- With "exact" I only get a cappella compositions

What I would like is to exactly include voice solo soprano and bass, to exactly exclude all other voices and get duetts a cappella + duetts with pianoforte + duetts with lute + duetts with guitar + duetts with .....