Scoring: IMP
You have an uninterrupted relay auction in which South opens a strong club, North shows 3334 with 13+ HCP and 5 AK controls, South makes a natural quantitative invite (5NT!) and North raises to 6NT.
Trick 1 goes ♠x-x-x-J. Clearly, something strange has happened on this trick. If LHO underled the ♠K, we now have at least 12 tricks. Or maybe RHO ducked the ♠K, which might have been necessary if we held Jxxx KQx Axx AKx, but on this layout, gives us extra squeeze chances if he also has long hearts. How do you judge the relative likelihood of these errors? How do you proceed?

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