code: public function addDiscountAdjustment(Adjustment $discount): void
      {
          foreach ($this->discounts as $d) {
              if ($d->isSameAs($discount)) {
                  $d->addAmount($discount->getAmount());
  
                  return;
              }
          }
  
          $this->discounts[] = clone $discount;
      }
docs: Adds the discount adjustment.
  
  @param Adjustment $discount