code: func (q *Quantity) Add(y Quantity) {
  	q.s = ""
  	if q.d.Dec == nil && y.d.Dec == nil {
  		if q.i.value == 0 {
  			q.Format = y.Format
  		}
  		if q.i.Add(y.i) {
  			return
  		}
  	} else if q.IsZero() {
  		q.Format = y.Format
  	}
  	q.ToDec().d.Dec.Add(q.d.Dec, y.AsDec())
  }
docs: // Add adds the provide y quantity to the current value. If the current value is zero,
  // the format of the quantity will be updated to the format of y.