(;GM[1]FF[4]CA[UTF-8]SZ[19]AB[ab][bb][cb][db][eb][sd][rd][qd][pd][od][oc][oa][ob][sq][rq][qq][pq][pr][ps][bp][cp][dp][ep][fp][fq][fr][hr][bn]AW[ac][bc][cc][dc][ec][fb][fc][pa][pb][pc][qc][rc][sc][or][oq][op][pp][qp][rp][sp][bq][cq][dq][eq][er]C[upper left: white to kill
upper right: black to kill
lower right: white to make a ko
lower left: black to make a ko];B[jj];W[ea];B[da];W[ba]C[easy!];B[rb];W[ra];B[sb]TR[qb]C[White can't approach due to shortage of liberties];W[rs]C[Due to the extra liberty at p1, white can't catch black in shortage of liberties. ];B[rr];W[qs](;B[ss];W[sr]C[Ko];B[br];W[ar];B[cr];W[bs];B[cs];W[es]C[A thousand year ko.
Black needs to play at the marked square after winning the ko to turn it into a direct KO.
White needs to approach at the marked square before starting the ko. 
--> few will start the ko, hence "a thousand year ko"]TR[dr])(;B[tt];W[ss]C[bent four in the corner, black needs to start the ko]))