code: public function add(AbstractQuantity $quantity)
      {
          $convertedQuantity = $quantity->to($this->getUom());
  
          return new static(
              $this->getAmount()->add($convertedQuantity->getAmount()),
              $this->getUom()
          );
      }
docs: Add
  
  Add two quantities together, keeping current Uom
  
  @author Christopher Tatro <c.m.tatro@gmail.com>
  @since  1.2.0
  
  @param AbstractQuantity $quantity
  
  @return Quantity