opt-in ReplaceByFee: nSequence can vary per input?

[Mike Lee Torris]

The recently added opt-in ReplaceByFee feature uses the nSequence number as a flag to indicate if the transaction is subject to RBF or not (or well, intended to be, in the end it's still up to individual nodes and miners to apply their own replacement policy).

However, in a Bitcoin transaction, each input has its own nSequence.

What nSequence is supposed to be interpreted as the RBF flag? The first input? The last? The largest? Or is it sufficient if the nSquence for just any one input (or more, or all) marks the tx the entire tx as RBF?

[achow101]

Any input's nSequence that is less than MAX_INT - 2 is sufficient to mark the entire transaction as replaceable.