there while Bob waits for a valid txn. Alice has a tough choice.
She can tell Bob she doesn't want the goods any more, and leave. In a few minutes the next block will get mined and
Alice will have paid for goods she didn't receive.
Alice can send another legit txn and purchase the goods. In a few minutes the next block will get mined and Alice will
have paid twice.
Any form of double spending is susceptible to double-paying.
