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2) In some auctions a cue bid could promise the ace and deny a lower ranking ace. For example, 1♠ - pass - 3♠ (limit raise) - pass - 4♦.
Not only that but there are further inferences to be assumed. Suppose it went:
1H-3H (limit)
4D-4S
From this sequence it can be inferred that partner controls clubs, most likely a singleton on this bidding, else he wouldn't have moved past 4H.

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