code: func (b *Adaptor) DigitalWrite(pin string, val byte) (err error) {
  	if strings.Contains(pin, "usr") {
  		fi, e := sysfs.OpenFile(b.usrLed+pin+"/brightness", os.O_WRONLY|os.O_APPEND, 0666)
  		defer fi.Close()
  		if e != nil {
  			return e
  		}
  		_, err = fi.WriteString(strconv.Itoa(int(val)))
  		return err
  	}
  	sysfsPin, err := b.DigitalPin(pin, sysfs.OUT)
  	if err != nil {
  		return err
  	}
  	return sysfsPin.Write(int(val))
  }
docs: // DigitalWrite writes a digital value to specified pin.
  // valid usr pin values are usr0, usr1, usr2 and usr3