I've now got a working producer. I've implemented my own Candidate class, reco::V0Candidate, which will be included in CVS soon under DataFormats/V0Candidate. This was after long correspondence with Luca Lista.

I'm now starting work on making the producer take a generic TrackCollection and loop over the tracks to find the candidate oppositely-charged tracks to pass to the KalmanVertexFitter. We have various ideas about how to deal with cutting down the bad tracks that won't produce a good vertex, but I'm first going to implement the looping functionality. This is actually kinda nontrivial at this point, although I'm sure it's easier than I'm thinking.