It is possible to deduce what they might be attempting to form in their hands by keeping track of the cards they draw and discard. An opponent may be nearing the completion of a golds rummyset or sequence involving those particular cards if, for example, they routinely draw cards from the discard pile rather than the deck. Also, observing trends in their gameplay can help you adjust your approach. It could indicate that a player is working toward a particular meld if they have a tendency to hold onto some suits or ranks longer than others.
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