code: func (q *Quantity) Neg(y Quantity) error {
  	switch {
  	case y.Amount == nil:
  		*q = y
  	case q.Amount == nil:
  		q.Amount = &inf.Dec{}
  		fallthrough
  	default:
  		q.Amount.Neg(y.Amount)
  		q.Format = y.Format
  	}
  	return nil
  }
docs: // Neg sets q to the negative value of y.
  // It updates the format of q to match y.