code: @Override
      public boolean canLeave() {
        if (reached == 0) {
          return false;
        }
        if (namesystem.now() - reached < extension) {
          reportStatus("STATE* Safe mode ON.", false);
          return false;
        }
        return !needEnter();
      }
docs: Safe mode can be turned off iff the threshold is reached and the extension
  time have passed.
  
  @return true if can leave or false otherwise.