Add existing (pending) order back to cart

When a customer has gotten to the payment page (credit card), but then failed to pay, a WC order (status=pending) is created (in wp-admin > WC > Orders)

If the customer returns to the site later, logs in and then wants to finish paying for the order – how can I re-add this order back to the cart programmatically (without creating a new order)?

thanks

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