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this isn't it

have you never heard or read someone start a reply with “i agree” or “i disagree”? in any case, what an awfully off-topic, pedantic thing to get bent out of shape about.

I understand “This” below a comment as a sort of a sign that it messages exactly what the replying person wanted to say. Akin to inheritance. If you just reply with an extension, it may not be as clear that you fully support it.