code: func MustParse(str string) Quantity {
  	q, err := ParseQuantity(str)
  	if err != nil {
  		panic(fmt.Errorf("cannot parse '%v': %v", str, err))
  	}
  	return *q
  }
docs: // MustParse turns the given string into a quantity or panics; for tests
  // or others cases where you know the string is valid.