DaviidM: If the attacking player plays an empty triangle after playing kosumi then the defender is dead. Connecting the group does not create a seki because the attacker can make a block of four; if the defender prevents that then the attack can play on the outside killing the stones.
Nicolas: (lower right corner) black O1 white Q1 black R3 white P1 black S1 is marked as incorrect but it works (white is in atari and has to play Q2, then black O2)
If the attacking player plays an empty triangle after playing kosumi then the defender is dead. Connecting the group does not create a seki because the attacker can make a block of four; if the defender prevents that then the attack can play on the outside killing the stones.