Net versus ladder
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[problem- can't this net be defeated by the sequence c-d-e-f-g? sorry for poor notation.]
No, see here "White can't cut...". Black plays at g instead of f. White has no choice but to capture at f; B at h forces white to fill in at and a couple more black moves capture the whole thing.
The net is better, because the ladder variation allows white to play a ladder block somewhere along the path of the ladder
Tapir: Even if there is no ladder breaker played by White, Black needs to capture at a time just in case. That is what I took from Kageyama: to capture with a ladder you need two moves, to capture with a net only one.
In this variation of the taisha joseki. What the books say is that is only playable when the ladder at a is good for White, capturing the stone with the net White b and c isn't enough.
If the net continuation were good, White could play at in the setup diagram independent of the ladder, thereby avoiding the most difficult variations.
After these plays, however, Black has a good result, and the possibility of a black peep at d remains. Also forces . Both of these effects are there because the marked black stone is still on the board.
For comparison, see the usual result from the ladder variation. White's capture in the ladder forces .