code: public function getShipments($filter = null)
      {
          if (null === $filter) {
              return $this->shipments;
          }
  
          return $this->shipments->filter(function(ShipmentInterface $shipment) use ($filter) {
              return $filter xor $shipment->isReturn();
          });
      }
docs: Returns the shipments.
  
  @param bool $filter TRUE for shipments, FALSE for returns, NULL for all
  
  @return \Doctrine\Common\Collections\Collection|ShipmentInterface[]