Wocoomerce - 为订单设置折扣

Question

我正在使用wc_create_order()以编程方式创建订单，而我无法找到为订单设置折扣的方法，例如

$order     = wc_create_order();
$sub_total = $_POST['totals']['totalBefore'];
$discount  = $_POST['totals']['discount'];

$order->set_discount( $discount ); //this function doesn't exit, need something to do that work
$order->set_total( $sub_total - $discount );  

知道我是怎么做到的吗？