this may highlight a bug in the library

game.sgf

(;GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2]
RU[AGA]SZ[19]KM[7.50]
GN[NAGF YoderChampagnie 3rd game]PW[Remi Champagnie]PB[Eric Yoder]WR[6d]BR[6d]PC[https://online-go.com/review/900806]C[This is the legendary broken ladder game in the semi finals of this year's pro qualification tournament. Eric Yoder, who pulled off a broken ladder game against me in this year's Midwest Open, once again shocked the world with another broken ladder. In the most dramatic way possible, Eric turned a desperate game into a victory and advanced to the finals, ultimately placing third and just missed out on professional promotion. My commentary will focus on the broken ladder part of the game.

The original SGF contained two sequence errors, which I have fixed here.]RE[B+5.50]
;B[pd]
;W[dp]
;B[pp]
;W[dd]
;B[cn]
;W[nc]
;B[cf]
;W[qq]
;B[qp]
;W[pq]
;B[op]
;W[nr]
;B[fc]
;W[df]
;B[dg]
;W[ef]
;B[cd]
;W[cc]
;B[ce]
;W[dc]
;B[ic]
;W[hd]
;B[hc]
;W[di]
;B[eg]
(;W[fg]
;B[fh]
;W[eh]LB[de:A]TR[fg][eh]C[Here Remi made some overplays. The triangled moves are risky because White has an obvious weakness at A.]
;B[ch]
;W[ci]
;B[ff]
;W[gg]
;B[de]
;W[ee]
;B[ed]
;W[fe]
;B[ge]
;W[gf]
;B[fd]
;W[ff]
;B[bc]TR[ee][fe][df][ef][ff][gf][fg][gg]C[Now White is beat up like a dumpling on the outside, and the corner is in danger.]
;W[bg]
(;B[cg]LB[bh:A]C[White's peep is a tricky move. Black cannot block at A.]
;W[bb]
(;B[ad]C[This tiger's mouth is a huge blunder. Black lost 8 points with this move.]
;W[bh]
;B[ab]C[Now the game is suddenly very good for White. It's hard for Black to capture the corner unconditionally.]
;W[ca]
;B[bf]LB[gh:A][fi:B]C[Blocking here is a slow move. Black needs to fight at A or B. It is not good enough to simply capture the corner.]
;W[cl]LB[gi:A]C[It's better for White to reinforce at A.]
;B[af]LB[aa:D][ad:A][af:C][bf:B]C[Black has spent three moves in the corner, but White can still make a ko with D.]
;W[gi]C[The game is favorable to White now, since Black made several mistakes in the top left. White leads by about 3 points, according to AI.]
;B[cq]
;W[cp]
;B[dq]
;W[ep]
;B[bp]
;W[bo]
;B[bq]
;W[bn]
;B[fq]
;W[qc]
;B[pc]
;W[qd]
;B[pe]
;W[pb]
;B[ob]
;W[qb]
;B[nb]
;W[qf]
;B[mc]
;W[go]
;B[he]
;W[gq]
;B[fp]
;W[fo]
(;B[gr]LB[eq:A]C[Hane here is the wrong shape. It's better for Black to connect at A.]
;W[gp]
;B[fr]
;W[pl]
;B[rq]
;W[rr]
;B[or]TR[rq][or]C[Here starts the famous broken ladder variation. At this point White is already ahead by 6 points, so while the game is still far from over White is clearly leading. The triangle move combo usually requirea a favorable ladder for Black, but in this game Eric is planning on doing a large sacrifice.

Remi must be confused about what Black is trying to do here, but I'm sure Eric has thought everything out at this point.]
;W[oq]
;B[nq]
(;W[np]LB[pr:A]C[In hindsight White could have atari at A first to avoid the ladder variation.]
;B[mq]
;W[pr]
;B[no]
;W[mp]C[On paper the ladder works for White, but...]
;B[lp]
;W[mo]
;B[gh]
;W[hh]
;B[fi]C[Black can create potential ladder breakers in the top left.]
(;W[fj]
;B[gj]
(;W[hi]LB[ei:A][gk:C][mn:B]TR[gj]C[If White captures at A, then Black can proceed with the ladder at B. The triangle stone makes the ladder favorable to Black.

After White connects, the broken ladder trade is inevitable. 

The best move for White now is actually C.]
;B[mn]C[Time to get popcorn and enjoy the show!]
;W[lo]
;B[ko]
;W[ln]
;B[lm]
;W[kn]
;B[jn]
;W[km]
;B[kl]
;W[jm]
;B[im]
;W[jl]
;B[jk]
;W[il]
;B[hl]
;W[ik]
;B[ij]
;W[hk]
;B[hj]
;W[gk]
;B[fk]
;W[gl]
;B[ej]CR[hl][kl][hm][im][lm][hn][in][jn][mn][ho][io][jo][ko][no][hp][ip][jp][kp][lp][mq][nq]LB[fl:A][gm:B]TR[ee][fe][df][ef][ff][gf][fg][gg][hh][gi][hi]SQ[je][ke][le][me][jf][kf][lf][mf][jg][kg][lg][mg][jh][kh][lh][mh]C[Through a brilliant broken ladder sequence, Black has effectively captured white's dumpling and erased most of the potential on the left side. While White can certainly get a lot of points on the right and bottom, it wouldn't scale to Black's gains. 

I will discuss in more detail why the trade is good for Black.

Before we dive into that, I will quickly mention that Black should have made the A/B exchange before capturing here, since it reduces White's points in sente. Capturing here without the atari exchange was a 4 point mistake, so Black is ahead by 6-7 points rather than 10-11.

If we account for the potential territory in the square regions, Black has gained about 40-50 points in the top left, while erasing at least 10-15 points of White's territory. In addition, White's center potential is destroyed. 

White's gain is harder to estimate, but White probably has around 30 points around the circle area plus some potential on the right side. White can conceivably get 50-55 points total, but Black has already gotten about 50 points, so White is effectively a move behind in terms of getting territory. Further, Black's center can grow even bigger if White spends all his moves developing the right side.
]
(;W[kk]LB[jj:A]C[This atari is a big mistake. White should atari at A instead.]
;B[jj]
;W[ll]TR[ih][ii]C[Black gains all the points around the triangle area.]
;B[og]LB[li:A]C[The keima at A would be the right direction. In this game Black's territory in focused on the top left, so Black should approach from the left.]
(;W[iq]LB[lh:B][ql:A]TR[hl][im][jn][ko][lp]SQ[jp][kp][kq][lq][mq][nq][kr][lr][mr][ks][ls][ms]C[White made a critical mistake of misjudging the values of the ladder stones. While geting the entire bottom side is very big, we have to consider the marginal benefit of this move. Even if White doesn't spend a move to reinforce the bottom, the five stones in the center will most likely still be dead. Thus, White's move here is really about points in the rectangle region, which is only about half of the bottom territory. 

Now we can see that both capturing the bottom right with A or reducing at B are actually way bigger than this jump. The game move is closer to a big endgame than an urgent point in the midgame.

After this mistake White found no chances. ]
;B[qg]
;W[rf]
;B[li]C[Black removes aji in the center with this move, and White has no chance from here.]
;W[nj]
;B[oi]TR[jd][kd][ld][md][ee][ie][je][ke][le][me][ne][df][ef][ff][gf][hf][if][jf][kf][lf][mf][nf][fg][gg][hg][ig][jg][kg][lg][mg][ng][hh][ih][jh][kh][lh][gi][hi][ii][ji]SQ[id][nd][od][oe][fj][hl][kl][gm][hm][im][lm][mm][nm][om][pm][qm][rm][sm][gn][hn][in][jn][mn][nn][on][pn][qn][rn][sn][ho][io][jo][ko][no][oo][po][qo][ro][so][ip][jp][kp][lp][op][pp][qp][rp][sp][jq][kq][lq][mq][nq][rq][sq][kr][lr][mr][ks][ls][ms][ns]C[Black's center swelled to 55 points, not including the points gained from eliminating the ko aji in the corner or reducing White's left side. Accounting for everything, Black got about 70-75 points from the trade.

In contrast, White can get at most 70 points if he manages to kill the bottom right side. In the meantime, Black will profit more from making beneficial exchanges and taking sente endgame. We have to also remember that in the game before the broken ladder exchange Black already had more points. White needed to gain more points than Black in this trade because Black has already secured some points in the top right while White got influence in the center. 

]
;W[qk]
;B[ir]
;W[jr]
;B[hr]
;W[kq]
(;B[qi]LB[qm:A]C[Black can exploit the aji in White's territory with A, but as mentioned before Black can win the game even if White kills the entire bottom right, so Eric is playing it safe.]
;W[mi]
;B[mh]
;W[lj]
;B[ki]
;W[fl]LB[qm:A]TR[ad][ej][gr]C[Once again, Black can play at A and make something happen in White's territory, but Eric was confident that he has enough, which is correct. Black went on to win by 5.5 points.

I think White's main issue was that he didn't evaluate the position accurately after the broken ladder trade and played too passively as a result. White allowed Black to develop the top side too efficiently, and lost a favorable game. Eric's genius is on full display this game. He took a lot of risk in devising this broken ladder strategy, and he was rewarded for that in the end. If he improves his overall play consistency (avoiding the triangle mistakes) he will be a top contender for the professional title. 


No further comments.
]
;B[rj]
;W[rk]
;B[el]
;W[em]
;B[dl]
;W[dm]
;B[ck]
;W[dk]
;B[ek]
;W[bk]
;B[dj]
;W[bj]
;B[ni]
;W[ei]
;B[fj]
;W[mj]
;B[sk]
;W[sl]
;B[sj]
;W[rm]
;B[pj]
;W[pk]
;B[oj]
;W[ok]
;B[pf]
;W[rg]
;B[rh]
;W[sg]
;B[ah]
;W[sh]
;B[hg]
;W[hq]
;B[qe]
;W[re]
;B[cj]
;W[eq]
;B[er]
;W[bm]
;B[ai]
;W[bi]
;B[ag]
;W[ap]
;B[aq]
;W[ao]
;B[aj]
;W[ak]
;B[pa]
;W[qa]
;B[oa]
;W[qj]
;B[ri]
;W[is]
;B[hs]
;W[js]
;B[kj]
;W[dh]
;B[si]
;W[]
;B[]
;W[])
(;B[qm]
;W[ro]
;B[rk]
;W[rl]
;B[ql]
;W[rm]
;B[pk]
;W[qj]
;B[ok]C[Black can do an effective reduction like this and would be ahead by nearly 20 points.]))
(;W[lh]C[This is White's last chance. ]
;B[jh]LB[ql:A]C[Very likely White can reduce in sente and would be able to kill the bottom right with A.]))
(;W[jj]
(;B[hg]C[Black would still allow White to ponuki in the center, except White will have more potential on the right.]
;W[kk]C[White still has a chance in this game.])
(;B[kk]C[Black cannot connect here.]
;W[ii]
;B[fj]
;W[if]
;B[ji]
;W[jh]
;B[hg]
;W[ki]
;B[hf]
;W[nn]C[Black is in trouble here.])))
(;W[ei]
;B[mn]C[White dies. Though Black made some bad exchanges in the top left, killing the three stones in the bottom right is orders of magnitude bigger, since it strengthens Black's only weak group.]
;W[lo]
;B[ko]
;W[ln]
;B[lm]
;W[kn]
;B[jn]
;W[km]
;B[kl]
;W[jm]
;B[im]
;W[jl]
;B[jk]
;W[il]
;B[hl]
;W[ik]
;B[ij]
;W[hk]
;B[gk]C[White dies.])
(;W[gk]
;B[hi]
;W[ei]C[Both ladders are favorable to White.]
;B[mn]
;W[lo]
;B[ii]C[Black can still force a trade like this, but it would be a better trade for White than the game.]
;W[gi]
;B[ko]C[Black is ahead by 2-3 points in this diagram, rather than 10 points in the game.]))
(;W[mn]
;B[hg]C[A trade like this is good for Black, since there is now no aji in the top left and Black's group on the bottom right is still far from dead yet.]))
(;W[pr]
;B[mr]
;W[np]
;B[mq]
;W[no]C[White will still maintain his lead in this variation. ]))
(;B[eq]
;W[gp]LB[co:A]C[Black gets sente now, and later there will be aji at A.]))
(;B[ei]
;W[dh]
;B[bi]C[Black should make exchanges on the outside first.]
;W[fi]
;B[ad]
;W[gh]
;B[bj]C[Compared to the game, White's corner is still close to dead, but White does not have much influence on the outside.

There's a ko aji in the corner still, but even if White lives in the end Black still has a lot of points on both sides.]
;W[eb]
;B[ab]
;W[ca]
;B[fb]
;W[aa]C[White can make a ko like this.]))
(;B[bh]
;W[cg]
;B[dh]
;W[bf]C[Black falls apart after White turns here.]
(;B[be]
;W[ae]
;B[bd]C[It seems like Black can win the semeai, but...]
;W[ah]
;B[bi]
;W[bj]
;B[ei]
;W[ai]
;B[eh]
;W[bb]LB[af:A][ag:B]TR[ah][ai][bj]C[Due to the triangled stones and liberty shortage in the corner, Black's group in the corner is dead. While there usually throw in at A or B, ])
(;B[ae]C[This jump also doesn't work.]
;W[ei]
;B[ag]
;W[bi]
;B[ah]
;W[bb]C[White got both sides, and now the corner is alive.]
;B[ac]
;W[ab]
;B[be]
;W[eb])))
(;W[fd]C[This is the vital point in the fight. Now White has no shape weaknesses.]
;B[ch]
;W[fg]
;B[fh]
;W[gg]C[It'll still be a relatively peaceful game.]))