Allow less than the total number of tracking detectors to build a track
At the moment all tracking detector layers are used to build a track. In case of two or more hits in at least one detector, all possible combinations of hits are tested (by picking one hit per detector) and the one yielding the track with the best chi squared is chosen.
This can bias the segment in case one of the tracking detectors is not efficient for the event but has a noise hit. In this case, the track chi squared would be better if the segment were build without the noise hit included. To solve it we need to allow also creating combinations involving less than the total number of tracking detectors, as long as it is larger than 3.
The impact of this bug is at the moment not quantified.