Quick ruling Question?

[OutOfCrown]

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blinds at 300/600, player A limps for 600, player B raises to 3000 and player C reraises all in for 4,200.

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Now that we've established that A can raise, I do have a follow-up question. What is the minimum raise amount for player A?

My guess is that the underraise really functions (as someone else said) as a "call plus some extra chips", in which case I would expect that the minimum raise would be calculated off of B's raise (as if C had "just called").

But this has always confused me and I can't find a clear explanation of it in Robert's (would appreciate pointer if it is in there)