function addToCart(state, _ref) {
	  var product = _ref.product;

	  return state.hasIn(['itemQty', product.id]) ? state.updateIn(['itemQty', product.id], function (quantity) {
	    return quantity + 1;
	  }) : state.setIn(['itemQty', product.id], 1);
	}
CartStores holds the mapping of productId => quantity
and also maintains rollback information for the checkout process