code: func (l *logger) Debugf(format string, vals ...interface{}) {
  	l.mut.Lock()
  	defer l.mut.Unlock()
  	s := fmt.Sprintf(format, vals...)
  	l.logger.Output(2, "DEBUG: "+s)
  	l.callHandlers(LevelDebug, s)
  }
docs: // Debugf logs a formatted line with a DEBUG prefix.