Yeah, that's it!
A simple retractor problem
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Unfortunately it's not unique, since retracting exf8=R (all four possibilities exQf8=R, exRf8=R, exNf8=R, exBf8=R) also works.
I've posted a similar thing before, which might be of interest as well.
White might have given mate on the last move.
White now to retract his move, and mate at once.