One problem with green addresses is that there is no punishment for double-spending within the Bitcoin system itself. To solve this, you decide to design an altcoin called “GreenCoin” that has built-in support for green addresses. Any attempt at double-spending from addresses (or transaction outputs) that have been designated as “green” must incur a financial penalty in a way that can be enforced by miners. Propose a possible design for GreenCoin.