I don't see a way to save all the black stones without Ko. --Anonymous
Tapir: The first three diagrams are correct imho (and the first is the intended solution). With the 2nd one showing absolute failure (that is big life for black). Small life is the best for both - if black isn't ko monster the ko is a failure. The fourth diagram is still a simple ko after ,,,. By recapturing , black may make an eye in the corner, but in the meantime white takes the three stones and destroys the other eye on the top.
A ko is 38 points big in my count, but black needs enough threats to answer even quite small threaths (worth only 10-14? i'm not sure here) to gain from it in comparison to small life.
jvloenen: is nothing but an internal ko threat. as shown in this diagram isn't correct. It should retake the ko first. As this is a picnic ko for white, black cannot play this ko in general. Therefore the first diagram is the correct solution.