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