Hand: 111222333m11p79s, we do double riichi, then draw 1m, and Mortal declares kan.
It decreases value of the hand so much:
Without kan: Double riichi, Junchan, Iipeikou (6 han) (we can read sets as 123m, 123m, 123m)
With kan: Double riichi, Sanankou (4 han)
Such kans change sets interpretation, but for simplicity they are allowed online. That doesn't mean they are always good.
This case is very tricky and could have been missed. It may seem that having sanankou is always optimal, but it's not.
/6 log:
Hand: 111222333m11p79s, we do double riichi, then draw 1m, and Mortal declares kan.
It decreases value of the hand so much: Without kan: Double riichi, Junchan, Iipeikou (6 han) (we can read sets as 123m, 123m, 123m) With kan: Double riichi, Sanankou (4 han)
Such kans change sets interpretation, but for simplicity they are allowed online. That doesn't mean they are always good.
This case is very tricky and could have been missed. It may seem that having sanankou is always optimal, but it's not.