if(workingItemId == null)
throw new InvalidIdentifierException("Could not find working item identifier");
Cart cart = lookupCart();
if (cart.containsItemId(workingItemId))
cart.incrementQuantityByItemId(workingItemId);
else {
Item toAddItem = _catalogControl.getItem(workingItemId);
if (toAddItem != null)
cart.addItem(toAddItem);
}