code: func newLogger(c *Context) (log15.Logger, error) {
  	lvl := c.String(FlagLogLevel)
  	l := log15.New()
  	log.Println(fmt.Sprintf("Using log level %s", lvl))
  	v, err := log15.LvlFromString(lvl)
  	if err != nil {
  		return l, err
  	}
  	h := log15.LvlFilterHandler(v, log15.StreamHandler(os.Stdout, log15.LogfmtFormat()))
  	if lvl == "debug" {
  		h = log15.CallerFileHandler(h)
  	}
  	l.SetHandler(log15.LazyHandler(h))
  	return l, err
  }
docs: // Logger ==============================