(;GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2] RU[Japanese]SZ[19]KM[6.50] GN[Zipper Problem]PW[White]PB[Black]CP[© 2010 Fort Myers Go Club]GC[This work is licensed under the Creative Commons Attribution 3.0 United States License. To view a copy of this license, visit: http://creativecommons.org/licenses/by/3.0/us/ or send a letter to: Creative Commons 171 Second Street, Suite 300 San Francisco, California, 94105, USA.]SO[http://www.fortmyersgo.org/]AW[ap][bp][cq][dq][br][er][fr][ds]AB[cn][ao][bo][eo][cp][dp][gp][eq][fq][gr][fs][gs]C[Black to play.] (;B[ar] ;W[bq] (;B[bs] (;W[cs]C[CHOICE] (;B[dr] ;W[as]LB[dr:A][cs:4]TR[bs]C[Correct. This is a two-step ko. White also has a choice of seki by playing 4 at A instead if black has surplus ko threats (navigation solution).RIGHT]) (;B[es] ;W[dr]C[Wrong. White is alive.]) (;B[cr] ;W[dr]C[Wrong. White is alive.]) (;B[as] ;W[es]C[Wrong. White is alive.])) (;W[dr]C[CHOICE] (;B[cs]LB[dr:4][cs:A]C[Correct. This is a seki. White has also a choice of a favorable ko by playing 4 at A instead (navigate solution).RIGHT]) (;B[as] ;W[cs]C[Wrong. White is alive.]) (;B[aq] ;W[cs]C[Wrong. White is alive.]))) (;B[cs] ;W[bs]C[Wrong. White is alive.]) (;B[es] ;W[dr]C[Wrong. White is alive.]) (;B[dr] ;W[cr] (;B[bs] ;W[es]C[Wrong. White is alive.]) (;B[es] ;W[bs] ;B[dr] ;W[er]C[Wrong. White is alive.]) (;B[cs] ;W[bs]C[Wrong. White is alive.]) (;B[as] ;W[bs]C[Wrong. White is alive.]))) (;B[dr] ;W[as] (;B[es] ;W[cs]C[Wrong. White is alive.]) (;B[cs] ;W[cr]C[Wrong. White is alive.]) (;B[cr] ;W[cs]C[Wrong. White is alive.])) (;B[es] ;W[as] (;B[dr] ;W[cs]C[Wrong. White is alive.]) (;B[cs] ;W[dr]C[Wrong. White is alive.]) (;B[cr] ;W[dr]C[Wrong. White is alive.])) (;B[cs] ;W[bs]C[Wrong. White is alive.]) (;B[cr] ;W[bs]C[Wrong. White is alive.]))