I see. Also, what's "one-shot" here?
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I think one-chance is when there's three copies of a tile out or in your hand leaving only one possible for an opponent to have. So if you have a 1 and you see three 2s out, you know the chances are way slimmer they have the 2 and you'd throw the 1 hoping for the best.
That said, looking at the code, it doesn't look like detecting this is currently hooked up.