code: func QuantityFlag(flagName, defaultValue, description string) *Quantity {
  	q := MustParse(defaultValue)
  	flag.Var(NewQuantityFlagValue(&q), flagName, description)
  	return &q
  }
docs: // QuantityFlag is a helper that makes a quantity flag (using standard flag package).
  // Will panic if defaultValue is not a valid quantity.