IMP pairs; lead J♥; table result 4♠=
East on this hand was our friend from a couple of other threads, the club's equivalent of the Secretary Bird. North-South were two rubber-bridge players, without a CC. It was round 1, and as the players were removing their cards from the board, East asked "Your basic system, please". North replied "strong NT, and 2/1". South added, "Thanks for telling me, pard, I thought we were just playing strong and 5". The auction proceeded as shown, and ten tricks proved routine. However SB was not happy. He asked, "How did you know that 1NT was forcing?" South replied, "I assumed it was in a 2/1 system; how else can you bid?". "Director", called SB.
"You had UI from your partner's response that your basic system was 2/1", continued SB. "The auction period begins when either partner withdraws their cards from the board (Law 17A) and your partner had taken out his cards when you responded to my question. You therefore had UI during the auction from your partner's reply to a question, and when you bid over 1NT you chose from among logical alternatives one that could demonstrably have been suggested over another by the extraneous information. You only knew you were playing 2/1 from partner's reply, and if 1NT were not forcing, it would be automatic to pass it."
"I can see why rubber-bridge players don't play duplicate with dickheads like you around," replied South. "Surely we are entitled to know our basic system?"
"Not if it comes from UI", replied SB, "and rubber bridge players without convention cards that have undisclosed implicit understandings are not particularly welcome either. I have no qualms about pigging you over a technicality. Go and fleece your pigeons at rubber bridge."
The director arrived, and calmed matters a little. But how do you rule?