@Override
  public boolean isDirty() {
    //TODO RC: with mapped files we actually don't know are there any unsaved changes,
    //         since OS is responsible for that. We could force OS to flush the changes,
    //         but we couldn't ask are there changes.
    //         I think return false is +/- safe option, since the data is almost always 'safe'
    //         (as long as OS doesn't crash). But it is still a bit logically inconsistent:
    //         .isDirty() is supposed to return false if .force() has nothing to do, but
    //         .force() still _can_ do something, i.e. forcing OS to flush.
    return false;
  }
@Override
  public boolean isDirty()