In Dia. 6-15-1, if Black does nothing the position is a seki, but if
Black plays 1, the result is a thousand-year ko as shown in Dia. 6-15-2.
After capturing and connecting the ko to create a seki as in Dia.
6-15-3, Black can destroy this seki and create a new seki as shown in
Even though the position in Dia. 6-15-1 is almost unimaginable in
actual games, it must be considered when rules are studied. If the
position in Dia. 6-15-1 exists together with other black ko threats
that White cannot eliminate, then Black can destroy the seki, create a
thousand-year ko, and capture White. This position should of course be
resolved by actual play.