code: func ParseOnOff(parameterName, val string, defaultValue bool) (bool, error) {
  	switch val {
  	case teleport.On:
  		return true, nil
  	case teleport.Off:
  		return false, nil
  	case "":
  		return defaultValue, nil
  	default:
  		return false, trace.BadParameter("bad %q parameter value: %q, supported values are on or off", parameterName, val)
  	}
  }
docs: // ParseOnOff parses whether value is "on" or "off", parameterName is passed for error
  // reporting purposes, defaultValue is returned when no value is set