BTC链中的硬分叉和软分叉、双花攻击Fork A hard fork allows you to directly replace rules and code. A soft fork involves adding more code to account for new rules. 硬分叉：替换规则或代码，无法兼容旧版本； 软分叉：添加新的代码，可以兼容旧版本； Hard Fork Soft Fork Double Spending Attack 51% Attack
