  This study was started to know mysterious classicality of nuclei.
  This time, I found a new rule for decoherence. I used a model without chaos.
As a result, it was shown that not only the intersection of classical
trajectories but also branching of classical trajectories are needed for
decoherence. In other words, it was shown that interactions between a main
system and environments have to make enough branchings of classical
trajectories of the main system for decoherence.
