accumulation: don't advance good tracks

prism/src/explicit/AccumulationProduct.java

@@ -133,14 +133,17 @@ public abstract class AccumulationProduct<M extends Model,Component> extends Pro
 				newGoodTracks.clear(trackNr);
 			} else if (oldTrack == null) {
 				newTrack = null;
+			} else if (oldGoodTracks.get(trackNr)) {
+				newTrack = null;
 			} else {
 				assert oldTrack != null;
 				newTrack = updateTrack(modelFromStateId, oldTrack, accexp, weights, mc);
 			}
 			
 			// check whether the track is good
-			if(!newGoodTracks.get(trackNr)) {
-				newGoodTracks.set(trackNr, isGoodTrack(newTrack, accexp, mc));
+			if(oldGoodTracks.get(trackNr) || isGoodTrack(newTrack,accexp,mc)) {
+				newGoodTracks.set(trackNr);
+				newTrack = null;
 			}
 			
 			newTracks.add(newTrack);