lavalyn 17k again - maybe I should stop submitting these spoilers.
BonhommedeNeige: I'm only a beginner, but I've thought long about this, and I don't understand how between the first diagram and the one above, Black has placed 3 stones and White has placed only one.
Now White moves to and can kill the group of 2 at lower left to create more eyespace, then try to kill square group of 4 it encloses.
Some clarification please!
Bill: captures , saving the two Black stones. (Although that is not really necessary, as you may work out if you wish.) White cannot escape.
Also, it does White no good to capture the four Black stones. See below.
White dies with a bulky five shape.
Even if Black doesn't play again in this area ( elsewhere) White still dies with a squared four.
If White takes the vital point, she forms a seki. An attempt by Black to kill the group instead yields either a bent four or a twisted four both of which are live shapes. These are shown in the failure diagrams below.
unkx80: Adding one failure diagram.
Easco adding another failure diagram.
Tas: Would the four black stones in the middle of the second diagram be counted dead? They'll have to go before white can be killed.
Tirian: I don't believe so. But moreover, I don't think it matters. If White were to take the four prisoners, it would just mean four extra points of territory for Black.
Karl Knechtel: You are confused. ko is a failure for an unconditional kill (and also for unconditional life) only if it is relevant to the status of the group. Here, any number of isolated, simple kos on the outside of the group cannot help White, unless the outside Black stones are endangered in some way.
The reason is simple: suppose White "wins" the ko. She cannot resolve it by capturing anything, since there is nothing to capture; and resolving it by filling the ko is counter-productive. So White can only pass or play elsewhere, and has no way to stop Black from taking the ko. Eventually Black wins, and can fill the ko and continue capturing.
Of course, we don't have to do this: because White has no effective resistance to being captured, she is simply dead.
Consider these simpler examples with the above reasoning if you are still confused: