Wat is Cum-ex trading, wash-sale rule?

Cum-ex trading is trading shares around a dividend date so that a trade entered into on or before the dividend declaration date settles after the record date, used to generate claims for refunds of dividend tax by more than one party for the same dividend, or by parties who never received it. A wash-sale rule disallows a tax loss on the sale of a security when the seller acquires the same or a substantially identical security within a set period around the sale.