Just wire the outputs to the Qbar outputs of the flip-flops rather than to
the Q outputs. This yields the sequence 111, 110, 101, 100, 011, 010, 001,
000 (which we see by simply flipping the bits of the sequence of the Q's), and
it so happens that this is a downwards count.
It's possible to derive the fact that this is a downward count by
observing that this is the arithmetic negation minus one, etc; it's probably
easier just to write out the sequence and observe it.
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