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Some people have suggested repurposing OP_NOP3 as a new opcode, OP_CHECKSEQUENCEVERIFY.
What would the new opcode do? Why is it useful? Can't the signer choose the sequence field when they sign?
What does OP_CHECKSEQUENCEVERIFY (OP_CSV) do?
Answers
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OP_CHECKSEQUENCEVERIFY (a.k.a OP_RELATIVECHECKLOCKTIMEVERIFY) can do Relative lock times (used for sidechains and lightning).
It basically checks, if a inputs sequence number is smaller then the sequence threshold (1 << 31 = 2147483648), and if, it will compare the nLockTime against the sequence number of the input.
More info:
OP_CSV (BIP112) only constrains the nSequence value. The comparison of nSequence against time happens independently of OP_CSV's usage (BIP68). – Pieter Wuille – 2016-02-08T02:41:31.467