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?
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OP_RELATIVECHECKLOCKTIMEVERIFY) can do relative lock times (used for sidechains and lightning).
It basically checks, if an input's sequence number is smaller than the sequence threshold (1 << 31 = 2147483648), and if so, it will compare the
nLockTime against the sequence number of the input.