(;GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2]
RU[Japanese]SZ[19]KM[0.00]
PW[White]PB[Black]AW[dn][co][bp][dp][ep][gp][fq][br][cr][dr][gr][hr][bs][fs]AB[aq][bq][cq][dq][eq][er][fr][ds][gs]C[This is is the initial position depicted on the floor at Ichigaya Station .]
(;B[cs]C[Correct: Black makes the strange-looking sacrifice of two stones at 1.]
;W[es]C[White captures.]
;B[ds]C[Black recaptures.]
(;W[fs]C[Under Japanese rules, White can play back at the stone marked with a circle to recreate the original position. 

If Black deviates from this cycle of four moves, he will die. If White deviates then Black will live unconditionally in a conventional way. 

What does this mean? It is like a ko, but there is no special rule to prevent repetition. If this position occurred in a game then one side may choose to back down if he thought he could win by accepting a loss in this corner, otherwise the game would be declared void. 

However, taken as an isolated tsumego problem White cannot kill Black so Black is alive. This is an example of chosei, or eternal life, and is incredibly rare --- I think it has occurred a single-figure number of times in recorded play. 

According to senseis.xmp.net/?Chosei it is so rare that a Chinese Go text says that if one occurs: You should buy fish, vegetables, meat and wine And have a good party to celebrate yourself.])
(;W[cs]C[However, under AGA rules, as used in BGA tournaments, such a repetition of the position is forbidden by the superko rule.

If the position occurred in an actual game, then White would be at liberty to fight the superko, just as you would normally fight a ko, by playing a threat elsewhere on the board then returning to this corner if Black answered the threat.

If White does not fight the superko and plays here, for example, Black can live, as in the following sequence.]
;B[fs]
;W[hs]
;B[ar]
;W[es]
;B[as]
;W[ap]
;B[ds]))
(;B[es]C[Mistake: Black allows White to create a 'bulky five' shape, and dies.]
;W[cs]
;B[ar]
;W[jd]
;B[as]
;W[cr]))
