(;GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2]
RU[Japanese]SZ[19]KM[0.00]
PW[]PB[Black to Play]C[Just play through the moves to move 16 which is the problem
position. These do not represent a game sequence!]
;B[bp]
;W[bq]
;B[cp]
;W[cq]
;B[dp]
;W[dr]
;B[dq]
;W[cs]
;B[er]
;W[ap]
;B[bn]
;W[ar]
;B[fq]
;W[]
;B[fs]
;W[]C[This is the problem position. Black to play
and capture the corner.]
(;B[bs]C[CORRECT: this is the vital point to stop white
making a nose for his two eyes. Often when
the nose is a diagonal one it is hard to see
if it is a real nose or not. ]
(;W[ds]
;B[br]C[This clearly fails as black gets both possible nose
points.])
(;W[br]
(;B[es]C[Black has the clever move at E1 which prevents white
from playing D3 as it's self atari.]
;W[as]
;B[ds]C[Taking the black stone doesn't work.])
(;B[ds]C[FAILURE: Playing this move looks like a clever move.]
;W[es]C[But white can start a ko - even though it's two stages.
Killing white directly is clearly better.]
;B[]
;W[as]
;B[ds]
;W[]
;B[]
;W[es]
;B[]
;W[ds]C[Two stage ko (Pass moves are ignored ko threats)])))
(;B[cr]C[FAILURE: This looks like it might me ko...]
;W[ds]C[But white lives with D1]
;B[bs]
;W[br]
;B[es]
;W[as])
(;B[br]C[FAILURE: White just lives easily]
;W[bs]
;B[ds]
;W[cr])
(;B[as]C[FAILURE: Easy life]
;W[bs])
(;B[ds]C[FAILURE: This just reverts to the ko sequence,
with black having to play what was the correct move
next in the sequence.]
;W[es]
;B[bs]
;W[br]
;B[ds]
;W[]
;B[]
;W[es]
;B[]
;W[as]
;B[ds]
;W[]
;B[]
;W[es]
;B[]
;W[ds]C[Still a multistage ko.])
(;B[es]C[FAILURE: White has no problems in making his nose
and two eyes now.]
;W[bs]
;B[cr]
;W[ds]))
