(;GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2]
RU[Japanese]SZ[19]KM[0.00]
PB[Black to play and rescue 4 stones]AW[qn][rn][po][so][pp][qp][rp][oq][sq][nr][or][pr]AB[np][op][lq][nq][pq][qq][rq][mr][qr]C[Black to play and rescue 4 stones]
(;B[ps]C[If Black tries to fill a white liberty...]
;W[rr]C[... then White just plays atari.]
;B[qs]
;W[rs]C[Black is captured.])
(;B[ns]C[Filling this liberty ends the same way.]
;W[rr]
;B[qs]
;W[rs])
(;B[qs]C[This is sometimes a move that gains liberties.]
(;W[ps]C[White loses if he plays this side]
;B[ns])
(;W[rr]C[This liberty doesn't work for white]
(;B[rs]
;W[sr]
;B[ns]
;W[sp]
;B[ps]C[Black wins this race.])
(;B[sr]
;W[ss]
;B[rs]C[Note that this sequence which often traps
two white stones fails from lack of liberties.]
;W[ps]))
(;W[rs]C[So white has to play here,]
;B[rr]
;W[sr]
;B[ss]
;W[sp]
;B[ps]
;W[rs]C[This ends in a ko, so is a failure for black]))
(;B[rs]C[This move is the correct move threatening to make two eyes in the corner.]
(;W[ps]
;B[ns]
;W[sr]
;B[qs]C[Black wins the race])
(;W[sr]
(;B[ps]
;W[rr]
;B[qs]C[Both sides are self-atari for white])
(;B[ns]
;W[rr]
;B[qs]C[This move also works - it makes the value of ko threats here less.]))
(;W[rr]
;B[sr]C[This is two eyes already.])
(;W[os]
;B[qs]
;W[sr]
;B[ps]C[Not a ko shape])
(;W[qs]
;B[ps]C[No good.]))
(;B[sr]C[This also looks like it threatens two eyes.]
;W[qs]
(;B[rr]
;W[rs]
;B[sp]
;W[ps]
;B[ns]
;W[sq]C[But it ends in a bad ko])
(;B[sp]
;W[rs]
;B[ps]
;W[os]
;B[ss]
;W[sq]C[This also fails in ko.])))
