code: func (l *Log) createFiles(sev Severity) error {
  	now := time.Now()
  	// Files are created in decreasing severity order, so as soon as we find one
  	// has already been created, we can stop.
  	for s := sev; s >= InfoLog && l.file[s] == nil; s-- {
  		w, err := newFlushSyncWriter(l, s, now)
  		if err != nil {
  			return err
  		}
  		l.file[s] = w
  	}
  	return nil
  }
docs: // createFiles creates all the log files for severity from sev down to infoLog.
  // l.mu is held.