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To set the latch, we need to momentarily set S=0, and then flip it back to S=1.
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If we clear S, then a=1, and A=1.
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Since R=1, b=0. This makes B=0, which makes a=1.
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If we restore S=1, a will not change, because B=0.
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Therefore, the latch continues to store value 1 as long as S=1.
(In a similar way, we can clear the latch by momentarily setting R=0 and back to R=1.)
