我们不能安全地实现 OP_BLOCKNUMBER。如果网络分裂后发生区块链重组，交易必须仍能进入更晚的区块。OP_BLOCKNUMBER 交易及其所有依赖交易会失效。这对后来持有这些币、但并未参与限时交易的人不公平。
We can't safely do OP_BLOCKNUMBER. In the event of a block chain reorg after a segmentation, transactions need to be able to get into the chain in a later block. The OP_BLOCKNUMBER transaction and all its dependants would become invalid. This wouldn't be fair to later owners of the coins who weren't involved in the time limited transaction.
nTimeLock 的作用正好相反。它是一笔尚未锁定的交易，在截止期限前能不断被新版本替换。只有锁定后，它才能被记录到链上。期限到达时版本最高的那笔会被记录。举例来说，它能用来写一笔托管交易，自动永久锁定并完成——除非在期限前被撤销。这个特性尚未启用或使用，但支持代码已在，之后能实现。
nTimeLock does the reverse. It's an open transaction that can be replaced with new versions until the deadline. It can't be recorded until it locks. The highest version when the deadline hits gets recorded. It could be used, for example, to write an escrow transaction that will automatically permanently lock and go through unless it is revoked before the deadline. The feature isn't enabled or used yet, but the support is there so it could be implemented later.
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