(;GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2]RU[Chinese]SZ[19]KM[7.50]TM[1800]OT[25/600 Canadian]GN[John-Tromp-vs-Zen19-20120114]PW[Zen19N]PB[John Tromp]DT[2012-01-15]PC[The KGS Go Server at http://www.gokgs.com/]C[This is the second game from the man vs machine match between John Tromp and Zen19 (a computer program that plays Go - http://gogameguru.com/man-machine-showdown-board-game-go/).]RE[白胜18.50];B[pd]C[Tromp won the first game, raising the spirits of his supporters...];W[dp]C[but Zen won the next three games, winning the best-of-five match.];B[pp]C[I chose this game to review because I thought it was the most tactically interesting of the four.];W[dd]C[Since I'm expecting some people who are not serious Go players, but are interested in computer Go to look at this, I've gone into more detail than I normally would in a review. Please forgive me if you're a strong Go player and you feel like I'm pointing out the obvious at times :).];B[pj];W[jp];B[jd]C[So far everything is simple and peaceful.];W[np]LB[nq:A]C[This move is slightly unusual, but there's nothing wrong with it. White A is more common, partly because it makes it easier to enter the corner later.](;B[qn];W[qc]TR[jd][pd][pj]C[This move feels a bit early to me because if white lives in the corner, build a large influence very early in the game. However, it does follow the principle of invading a 4-4 point corner after black has extensions on both sides. The idea is that whichever way black blocks, one of the stones will become somewhat inefficiently placed.];B[qd]C[Blocking on this side is bigger...];W[pc];B[oc](;W[dj]LB[qc:A]C[If white was going to play here, it would've been better to do it instead of playing A. Making the exchanges in the top right settles the shape of the game and reduces white's options.](;B[od]C[This move is also ok. It's slow, but it's solid.](;W[gf]CR[dd][gf]TR[ed][fd][ef][ff]SQ[de][ee][fe][ge]C[This move feels strange because the shape made by white's two marked stones is brittle. I say that because there's at least one line separating them in both directions, which makes it easy to separate them.](;B[jj]LB[hc:G][gd:B][gf:A][if:F][jf:E][ig:D][ih:C]C[Playing this move at tengen (the center point) is insensitive to white's previous move at A. It would be better to play B, as in the variation, but if black wants to play in the center immediately, C or D would also be better because they prepare to resist white E at F (or F at E). However, then white might just play G, which is also very big.];W[pq];B[qq];W[jf]LB[jf:B][lp:A][pq:1][qq:2]TR[jj][pj]SQ[np]C[This move is natural now and it disrupts black's framework quite effectively. There's no particularly good local response for black, so it's better to play elsewhere.Playing the 1-2 exchange and then playing elsewhere just reduces white's flexibility yet again and makes white heavier. It would be good to apply pressure at A now because a fight here would make efficient use of black's two marked stones and could turn 1 into a bad move. In this case it's not really chasing white into your own framework because white's already played B.];B[kf]C[This move doesn't help the situation.];W[kg](;B[jg]C[Black probably played this to make use of tengen, but the price is too high...];W[ke];B[lf];W[je]TR[jd]C[Now black's marked stone has been damaged. White takes the lead.];B[ig];W[hg]C[This move looks strange and leads to bad shape, but it's quite strong in this position. I wouldn't have even considered this move, but the computer seems to specialize in this sort of thing.];B[if];W[ie]LB[he:A]TR[jj]C[Black's marked stone isn't even close enough to prevent a ladder, so black can't cut at A.];B[hh]LB[hf:B][gh:A]C[This is the best option, if white plays A, black plays B and cuts white.];W[hf]C[Bad shape, but necessary.];B[gh]LB[ih:A][kh:B][ii:C]C[It feels good to extend here and it looks like the shape move, but it leaves weaknesses in black's shape. White can play A or B later. Because of that it would be better to just play C here, but black may have felt that was too mild. Again you can see that tengen isn't working very hard...];W[le]C[This is good, white's going to build a huge area at the top. If white doesn't play here, black might.];B[mf];W[mm]LB[lp:A]TR[kk][ll][mm][nn][oo]C[This is a flexible move. It expands white's framework while reducing black's and also helps to cover the weakness around A. It has a nice feeling about it.The diagonal line marked with triangles represents the boundary (junction line) between black and white's frameworks. Notice how white expands onto the boundary, taking slightly more and leaving black with slightly less.];B[mk]C[Black doesn't let white puncture the framework a second time. The right side will probably become territory now, under normal circumstances.];W[me];B[nf];W[mc]LB[fc:A][cd:B]TR[pc][qc][pq]C[Except for the strange exchanges (marked), which may or may not become good later, white's playing very well so far. With this move, the top area has almost been 'completed'. If black doesn't do anything now white could play A (or possibly even B) and turn it into a huge territory. In addition, it's now possible to live without ko in the top right corner (see the variation).](;B[fc]C[This is good. The best time to invade a framework is just before it's turned into territory - note: if the other player can kill your invasion, it's already a territory ;). Waiting until the last minute and settling a group quickly reduces the efficiency of the other player's stones.](;W[hc]LB[mc:B][he:A]C[This move isn't very severe but it's calm, simple and good. It protects the cutting point at A using the stone at B (see variation).](;B[dc]LB[cc:A]C[This move tries to get more than just playing A and is often good. In this case though, I think A would be better.](;W[cd]LB[cc:B][hc:A]C[Unlike A, this move is just too soft. Black will settle the group easily now and white is losing the advantage. White should play at B instead (see the variation).];B[cc];W[ne]LB[ed:A]C[This move is sente, but there's no reason to play it now. White is wasting ko threats. Playing at A is big a vital point for both players, but black has to defend here first.];B[of];W[oe];B[pf]LB[oe:1][pf:2]C[When white pushed again here I thought maybe white was preparing to live in the corner and simultaneously create a weakness in black's right side. See the variation.](;W[pe]C[Scratch that. White just erased another possibility. This move is really clumsy. It's strange seeing a 'player' who is this strong make moves like this. The computer has a very different balance of strengths and weaknesses to a typical human player of the same level.];B[qe];W[ed]C[White comes back to take the vital point.];B[fb]LB[hc:A][cd:B]C[The result in the top left is good for black, because A and particularly B were soft. Black's regained back a lot of ground.];W[hm]LB[ih:A][im:B][dn:D][jp:C][oq:E]C[Zen style :). White can aim at A so you can't really fault this strategy. If black doesn't invade, white will get to complete a huge area at the bottom, so this move puts some pressure on black. Most human players following this strategy would probably move the stone one point to the right (to B) because it has a better relationship with C. White could also consider reinforcing the framework with D, cutting immediately at A or finishing the lower right shape with E.];B[fq]TR[dj][hm][mm][dp][jp][np]C[It's good timing to invade, again. If black gets a good result here he will make white's floaty center stones inefficient and take the lead. However, he has to tread carefully because this is white's playground and black can't expect too much. Look at all the white stones :)...];W[eq];B[fp];W[en]CR[dp]LB[dn:A][en:2][eq:1]TR[hm]C[The 1-2 combination used to be played a lot in ancient Chinese games. It's very severe, but it leaves more weaknesses in the corner than the alternative move at A (which is much more common these days). However, in this position this is a very nice move because it works well with White's H7 stone. I like it more than A.](;B[jq]TR[mm]C[I think this move is a little bit of an overplay. Black has to be careful because the marked white stone will make nearly all ladders good for white.];W[iq];B[ip]TR[hm][mm][en][np]C[White wants to play severely here, so... (see variation).](;W[kp]C[This move simplifies things, but letting off the pressure can be dangerous in this kind of fight. Black's overplay has paid off. Now black has room for a counterattack. This is the sort of situation where the player with better tactical and reading skills will take control of the game...];B[hq];W[ec]LB[fd:A][ih:B]TR[gh][hh]C[Very random. This move just wastes a ko threat and takes away white's liberties. It's actually quite a bad exchange because after white reduces her own liberties, it makes black's future move at A harder to defend against. That in turn makes cutting at B and threatening white's marked stones less effective than before.];B[eb];W[ir]LB[gn:C][gr:A][jr:B]TR[iq][ir]C[Now black has various forcing moves and possibilities. For example A threatens to capture the marked stones (by playing B next) and so do many other moves, so black is flexible. Black can wait and see how to best use the threat of capturing white's two stones. This is a critical time in the game to try to take control, so white is giving black a big opportunity.There's a variation starting with C that we'll refer back to a few times later.](;B[io]TR[iq][ir]C[With this move, black takes immediate aim at the marked stones (see the variation). It lacks guile, but it's not that bad.](;W[bc]LB[jr:A]C[White has to come back and play A at some point.];B[bb];W[ih]C[The timing for this cut seems a bit early.];B[jh];W[ii];B[ji](;W[hk]C[This is a clumsy move. It looks like white is trying to cut off black's lower left group indirectly, but this is giving up too much in advance.];B[hj]C[Too easy for black.];W[ik];B[ij]TR[io][fp][ip][fq][hq]C[By losing the option of cutting, white's just made a significant investment without any certainty of a return. It's a very risky way to play and the only way to justify these moves would be to kill black's marked stones now.];W[jr];B[er];W[dr]C[Black's group is getting a bit heavy.];B[ep];W[dq];B[eo];W[do];B[fn];W[em];B[go]LB[hj:2][ij:4][hk:1][ik:3]C[Black's alive for now, but white can still make indirect threats to kill. Black chose a painful way to live and helped white make a big territory on the left side, but since white also made the bad exchanges at 1-4 and black still lived, black gained a lot too. I feel it's still anyone's game.];W[gr];B[gq];W[ob]C[Another random exchange, unless white intends to make life in the corner right now, playing here just damages the top area.];B[nb](;W[nl];B[pk];W[lk];B[lj]LB[lk:2][nl:1]TR[fn][eo][go][io][ep][fp][ip][fq][gq][hq][er]C[It looks like 1 and 2 are trying to prepare an attack against black's marked stones. However, black can't be killed at the moment (see the variations to see why).](;W[oq]LB[qr:A]C[Playing at A is very big ~20 points counting endgame, but...];B[dh]C[Playing around here is also big.](;W[ch]C[This is the territorial approach.];B[cg];W[ci];B[dg];W[be]LB[fh:A]C[Interesting move. Maybe still aiming at A.];B[hb];W[ib];B[hd]LB[gc:A]C[This is sharp. Better than just playing A.];W[id];B[gc];W[ic](;B[ia]LB[pb:A]C[This move is a bit slow and greedy. Black's corner is already alive. It would be better to kill the top right corner at A.](;W[fe]LB[fh:A]C[This move is strange. White should do something about the big group at the top, but a strong human player would probably never choose this move because it doesn't gain much. Trying to look for its good points though, it does help slightly if white plays A later.];B[pb]C[Finally the situation in the top right is settled. White's stones are captured. Both players missed some chances here, but it all came out in the wash.];W[qr]C[This is big.];B[rp]LB[so:B][rr:A]C[This is a nice response because it means white can't play the monkey jump at B after extending to A.];W[rq];B[qp];W[pm]C[Good timing.];B[qm];W[pl];B[ql](;W[fj]LB[fh:A]C[If I were white, I'd still want to fight at A. I don't think white is leading by that much, but I wouldn't claim to be better at counting than a computer :).];B[ei]LB[fh:A]C[Black gets to defend against A.];W[fr]LB[kl:C][ll:B][lp:2][kq:1][hr:A]C[This is an interesting move. It threatens to kill black because A isn't sente anymore, which means white can simply answer 1 with 2 later. It's also a very big endgame move, so I'm impressed. Black should play something like B or C now to help the lower left group indirectly.](;B[bh]C[Black played here quite quickly without thinking. It's the losing move for black.];W[ja]C[Last chance to live at the bottom...];B[ha];W[gn]LB[fr:A]C[Bang! I was watching this game live and I'd been thinking about whether black still had any ways to live after A. The play progressed quite quickly after A though, so I was still reading when white played here and it was impressive to see a computer find the same sequence that strong observers were contemplating.];B[hn];W[gm];B[im];W[jn];B[in];W[hp](;B[kl];W[jm];B[jl];W[il]C[Black's dead and the game's over. Zen went on to win with a 3-1 score. It was a very interesting match and we got to see just how good computers are at Go now. Thanks John Tromp for putting up a good fight for the humans, Yoji Ojima (who created Zen), Hideki Kato (who provided the hardware that Zen ran on during this match) and Darren Cook for organizing this event and making the original Shodan Go Bet.In the endgame, Zen seemed to play lots of strange moves, but it was only because it was winning by so much. I'm not an expert in computer Go, but my understanding is that the MCTS algorithm looks for the move that has the highest probability of winning, based on thousands of playouts. When you have a large lead nearly any move that isn't an outright blunder has a high probability of leading to a win. The program may not care whether it wins by half a point or 100 points.Because of that, I haven't made any comments after this move, this is the last comment.If you want to find out more about this match and the history behind it, you can visit:http://dcook.org/gobet/and read my article at: http://gogameguru.com/man-machine-showdown-board-game-go/If you're interested in learning more about computer Go, subscribe to the computer Go mailing list at:http://computer-go.org/mailman/listinfo/computer-goI hope this commentary was helpful.David Ormerodhttp://gogameguru.com/];B[gd];W[he];B[bf];W[mb];B[ej];W[dk];B[ak];W[bl];B[al];W[bm];B[am];W[ab];B[cb];W[fk];B[ao];W[bn];B[an];W[rr];B[kn];W[jo];B[ll];W[ln];B[oo];W[nk];B[nj];W[ml];B[mj];W[ma];B[oa];W[jk];B[kk];W[bp];B[no];W[mo];B[ap];W[bq];B[aq];W[fi];B[fh];W[nn];B[ek];W[bi];B[ai];W[el];B[gk];W[ho];B[ef];W[bj];B[aj];W[di];B[eh];W[af];B[ag];W[ae];B[gj];W[cf];B[bg];W[gl];B[ff];W[ge];B[ee];W[fd];B[bk];W[ck];B[br];W[ok];B[oj];W[sp];B[so];W[sq];B[qo];W[cr];B[bo];W[co];B[on];W[om];B[km];W[bs];B[ar];W[ko];B[gi];W[fl];B[df];W[ce];B[pn];W[gg];B[fg];W[op];B[ba];W[nd];B[nc];W[de];B[aa];W[ac];B[ah];W[jb];B[as];W[cs];B[na];W[lm];B[];W[])(;B[jk]C[This move doesn't work anymore...];W[jl];B[il];W[gk];B[kl];W[jm];B[jo];W[kn];B[ko];W[ln](;B[hr];W[kq]C[Black dies.])(;B[kq];W[lp]LB[kq:B][hr:A]C[White can connect at A, so B doesn't threaten white anymore.]))(;B[jl]C[How about this one? It seems to me that, even with the benefit of plenty of time to read, this move doesn't work anymore either.];W[jk];B[jo](;W[kn];B[ko];W[ln];B[lp];W[kq];B[kk]C[Even if black cuts white off like this...];W[kl];B[ll];W[km];B[lo];W[mo]C[...black still doesn't have enough liberties to win a capturing race, so black dies.])(;W[ko]C[This would be a mistake...];B[km]LB[kk:B][kn:A]C[A and B are miai, as we discussed earlier.])))(;B[ll]TR[fn][eo][go][io][ep][fp][ip][fq][gq][hq]C[If black plays here, white can't kill the marked stones.]))(;W[fh](;B[fg];W[fi];B[gg];W[eg](;B[ef];W[eh];B[ff];W[df];B[ee]TR[cg][dg][dh]C[This move is strong and white's top group is threatened. Black's marked stones still have some potential too, so it's not that bad for black either.];W[ja];B[ha];W[kb];B[ac]LB[de:A]C[This move threatens to play A.](;W[cf])(;W[bd]C[White can't just connect...];B[de];W[cf];B[bf];W[ce];B[bg]C[Disaster.]))(;B[eh];W[ef];B[ej]C[This might also be possible, but remember that white is still aiming to kill black's group in the lower left.];W[fj]LB[gk:A];B[ek];W[fk]C[Black feels heavy.]))(;B[fi];W[ei]LB[fg:2][fi:1]C[White can cut like this, so playing 1 at 2 seems better.])))(;W[bg]LB[pb:A]TR[gf][hf][hg]C[It's better to take profit by connecting here. The top group can live independently or connect if it has to and the marked stones aren't really important anymore because black they don't separate any weak groups. Ideally white would like to hang tough at A instead, but it's complicated and difficult to evaluate in a game. A could lead to a capturing race in the top left.]))(;B[pb]C[This is a very big move now.]))(;W[be]LB[eb:2][ec:1][fd:B][fh:A]C[White can also prepare for a fight like this (trying to prepare a defense against black's push and cut at B). Note again how criminal the 1-2 exchange was earlier :).](;B[ac]C[If black answer here, which is big...];W[fh](;B[ei]CR[dh][ei]TR[fn][eo][go][io][ep][fp][ip][fq][gq][hq][er]C[Black would probably just play here. White can push black's two stones towards the lower left corner with the aim of engineering a double threat against both groups.])(;B[fi];W[ei];B[fg];W[eh];B[eg];W[dg]C[White's cut works.])(;B[fg];W[eg];B[fi];W[eh];B[ei];W[di];B[ef];W[dg];B[df];W[cg]C[This way works for white too.]))(;B[cj]LB[fh:A]C[Black can defend against A by playing like this and adopt a mutual damage approach. Maybe white didn't like this prospect...];W[ck];B[ci];W[ab];B[hb]C[It's a reasonable exchange for both.])))(;W[gn];B[hn];W[gm];B[im];W[jn];B[in];W[hp](;B[jk];W[jl];B[il](;W[gk];B[kl];W[jm];B[jo];W[kn];B[ko];W[ln];B[hr]LB[kq:B][fr:A]C[Black A and B are miai and black will get a second eye one way or the other.])(;W[hl]LB[kl:A]C[This way takes away a liberty, so black has to be careful. Black A won't work anymore.];B[jm];W[kl];B[km];W[lm];B[jo];W[kn];B[ko]LB[ln:C][kq:B][hr:A]C[Black still has A, B and C.]))(;B[jl]LB[gk:A][jk:B]SQ[ki][li][mi][mj][nj]C[Another way to live. Note that this move works even if white gets to play a stone at A in a later fight (so even though A breaks the variation starting with B, it's not sente to kill). The downside of playing this way is that black will lose more points in the marked area if white just lets black live and plays for a trade.];W[jk];B[jo](;W[kl]C[This is strong, but...];B[kn];W[km];B[ko]LB[jm:C][kq:B][hr:A]C[Black still has A, B and C.])(;W[ko];B[km]LB[kk:A][kn:B]C[A and B are miai.])(;W[kn];B[ko];W[lp];B[ln](;W[lm];B[km](;W[lo];B[jm]LB[kk:A][kn:B]C[A and B are still miai.])(;W[kl];B[lo]))(;W[km];B[kl]LB[kk:A][lm:B]C[A and B are miai.])))))(;W[pb]C[White could still live in the corner like this...];B[nc];W[mb];B[na];W[rb];B[rd];W[qa]))(;W[ij]C[...later, white wants to continue like this instead.];B[ik];W[hj];B[hk];W[gj]TR[gh][hh]))(;W[jm]C[If white blocks black in...];B[jr]C[This is what black was aiming for.];W[hr];B[gr];W[hp];B[is];W[gq];B[hs]TR[iq][hr][ir]C[White can't save the marked stones.]))(;B[gn]C[If black wants to play simply, he could force here to help settle the group. Note this sequence because black can still use it to settle the group after starting a fight in the corner.](;W[gm]C[We can expect that white will answer here.];B[jr];W[hr];B[gr];W[hp];B[io](;W[ho]LB[gq:A]TR[mm][io][ip]C[Normally white would have to capture at A now, but in this case the marked white stone let's white threaten to capture black's two stones in a ladder.](;B[er](;W[dr];B[in](;W[hn]C[White can resist like this...];B[gq]TR[iq][hr][ir]C[Black captures the marked stones, but...];W[jm]LB[dh:B][ih:A]TR[in][io][ip][iq][hr][ir]C[White can still enclose black because it's a capturing race between the marked stones. Now black needs to worry about the cut at A. Playing B could be a good way to defend indirectly.])(;W[gq];B[hn];W[kq];B[im]LB[fn:B][go:A]C[Black can escape comfortably like this. White can't cut around A because of shortage of liberties (black will capture some stones if white tries to cut). Black A is also sente to make two eyes, so black can still play this way in some of the other variations, even if there's already a white stone at B.]))(;W[gq];B[cq]C[This is good enough for black.]))(;B[hn]C[Black can't block here...];W[in];B[jo];W[jn];B[ko];W[lo]TR[mm];B[kn];W[km]C[Black's stones are captured.]))(;W[gq]C[This is what black is hoping for...];B[er](;W[dr];B[ho];W[kq];B[gp];W[kr];B[hq]C[Black is completely alive now and white's outside group is still thin.])(;W[ho]C[White can resist, but...];B[cq]C[Black can live in the corner.])))(;W[hr]C[If white tries to resist...];B[gm];W[gl];B[fl](;W[gk];B[em]C[This black group looks difficult to kill and white is ruining all of her potential territory.])(;W[fm];B[hl];W[gk];B[hk]LB[fk:B][im:A]C[A and B are miai. White's left side is still thin and black may be able to invade successfully later.])))(;B[cq]C[Since black is a bit more flexible now, black could play a probe in the corner and aim to settle the game quickly.](;W[er]C[White can fight like this, but it's not that easy...];B[gn]LB[do:A]C[Black can ask here first to make sure the group can be settled when needed. This exchange also helps with black's cut at A.];W[gm];B[do](;W[eo];B[ep](;W[dq];B[cp];W[cr];B[dn]TR[gn]C[See the value of the marked stone?];W[em]LB[dm:C][em:B][co:A]C[If white plays B at C instead, black will cut immediately. Black still needs to protect the cutting point at A.];B[dm]LB[el:D][cm:C][bn:B][co:A]C[This is a way to protect the cutting point at A. Instead of this move, black also has the shapes B and C to consider, but pushing here gives black the hane at D to look forward to.];W[bq]LB[br:A]C[Now white has to play here to stop black from playing at A.];B[bp];W[br];B[fr];W[es];B[jr]LB[el:C][gn:A][cq:B]C[Black can still use the sequence shown earlier (when black just played A instead of B). Black doesn't have to save all the stones on the left side because he already forced white to make two eyes in the corner. It will be enough to just settle a group there. Note that C will be a powerful move for black at some point.])(;W[cp];B[dq]C[Good for black.]))(;W[co];B[cp];W[eo];B[ep];W[dq];B[dn]C[White's losing control of the fight.]))(;W[cp];B[er](;W[dr]C[This is what black wants...];B[dq];W[cr];B[gr]LB[bq:B][jr:A]C[A and B are miai now.])(;W[dq]LB[gn:A][cr:C][gr:B]C[There are a few ways to continue now. Black could still play the sequence starting with A (shown earlier) and after black has B on the board he can look forward to playing C later.]))(;W[dq]LB[gn:B][cp:C][cq:A]C[If white plays here, black can treat A as a forcing move and make life for the group. Continuing with the variation at B (shown earlier) still seems good. Both players will try to get sente to play at C.])))(;W[hp]TR[fp][fq]C[Playing here looks like a strong move for white. Even though there's a proverb that says extend from the crosscut, this situation is different because white needs to consider the marked black stones too.](;B[io];W[hq]LB[kp:A]TR[mm][jp]C[The ladder at A doesn't work, so black will have trouble settling this group.];B[kq]C[Black has to fight like this.];W[jn];B[jo];W[kp]TR[io][jo][ip][jq][kq]C[It will be hard for black to manage both groups.])(;B[ir];W[io];B[hq]C[Black could try to resist like this. It might work well against Zen because it seems to have problems with fighting ko.]))(;W[hq]C[Just extending here follows the proverb about crosscuts, but it's too simple.];B[hp];W[kq]TR[jq]C[White can trap the marked stone, but...];B[jr];W[kr];B[jo];W[kp];B[hn]LB[gq:1][hr:3][ir:2][js:4]C[Black still has forcing moves like 1-4 and will settle this group easily.]))(;B[hp]C[This is a good move to start making eyeshape. It looks like black will be OK.])(;B[jr]C[This is also possible, but...](;W[iq]C[White might play this way...];B[ir];W[hr];B[hq];W[gq];B[hp];W[fr]LB[hp:A][gr:B];B[kp]C[Black has to be happy to fight like this.])(;W[jq]C[This white move is also possible.];B[ir];W[kr](;B[hq]C[Black should be able to settle the group like this.])(;B[hp]C[If black plays here...];W[iq](;B[hr];W[fr];B[gr];W[er];B[go]LB[go:A]C[Black's group isn't completely safe yet and white has closed off the corner while attacking. If black doesn't protect the vital point at A, white will strike there and black will be in serious trouble.])(;B[hq];W[gr]LB[iq:1][fr:B][gr:2][hr:A]C[White 1 and 2 are a 'famous' tesuji combination and white can play A or B next. Black's in trouble.])))))(;W[ob]C[If white lives now...];B[nb];W[pb];B[nc];W[mb];B[na];W[rd];B[re];W[rc]LB[qe:B][rf:A][kh:C]C[It leaves black with weaknesses at A and B - and don't forget about C.]))(;W[cc]C[White should hane here...](;B[cb]C[Black should probably try to settle the group like this.](;W[bb];B[eb];W[ca];B[ed];W[de]LB[ee:A][df:B]C[This is much better for white than the game. Black can escape by pushing at A, but white will just keep extenting with moves like B and make a large territory on the left side of the board.])(;W[db]C[White isn't strong enough in this area to try to kill black...];B[ec];W[bb];B[cd];W[bc];B[de];W[ed];B[fe];W[ce];B[fd]LB[bd:B][ff:A]C[...it just helps black escape. A and B are miai now and black is fine.]))(;B[cd]C[This crosscut is sometimes a strong move, but doesn't seem to work in this case...];W[bc]C[Extending here is strong.](;B[ce];W[de]LB[ec:A][cf:B]C[White A and B are miai.](;B[cf];W[ec])(;B[df](;W[ee]LB[ec:A][cf:B]C[This is a nice example of an 'empty triangle tesuji' :). A and B are still miai.])(;W[fd]LB[ee:A]C[This move doesn't work as well as A.];B[ec];W[cf];B[fe];W[ef];B[dg];W[gd];B[bf]LB[db:A][eg:B]C[If white plays A next, turning at B will be sente.])(;W[fe]C[This move doesn't work as well as A either.];B[ee];W[ed];B[fd];W[ef];B[ec];W[ee];B[dg])(;W[ec];B[ed]C[This is also annoying for white.](;W[db];B[ee])(;W[ee];B[eb]))))(;B[bd]C[Black can try playing here, but...];W[be]C[This is a tesuji.];B[ce];W[de]LB[ec:A][cf:B]C[A and B are miai.])(;B[hd]TR[dd]C[Black could try to engineer a double threat like this, but it doesn't seem to work well.];W[id];B[gd]LB[ic:A][dd:B]C[Black is aiming at A and B.](;W[de]C[White can simply play here, and...];B[ic];W[ib];B[jc];W[jb];B[hb]LB[ec:B][fe:A]TR[ib][jb]C[Black can't cut off the marked stones so white can play A or B next and black it looks difficult just to live.];W[fe];B[db];W[cb])(;W[hb]LB[ic:A]C[If white defends against A, then black's strategy pays off...];B[bd];W[be]C[This move doesn't work so well anymore.];B[de];W[ed];B[ce];W[ec];B[eb];W[db];B[ee]TR[dj]C[Black's happy to sacrifice a few stones and get out of a tight spot. White's marked stone is starting to feel like a target.])))))(;B[cc]C[If I were black I'd just invade the corner here, because white is very strong in this area and this makes it easy to settle black's group without complications.];W[dc]C[White would probably block here.];B[cd];W[de]LB[ce:A]C[White has enough stones around that white could also play at A here and invite black to start a complicated fight.];B[db];W[eb];B[cb];W[ec];B[cf]C[White takes sente.])(;B[he]C[If black cuts here...];W[ge];B[hd];W[gd];B[id];W[jc]C[the black stones can't go anywhere...];B[kd];W[kc];B[ld];W[md]))(;W[ec]LB[fd:A][fe:C][ef:B]C[It's tempting to play here. If black plays A, white plays B and it would be great for white. After that, even if black manages to find life at the top, white can live in the top right corner without worrying about the top area anymore. However, maybe Zen's assessment was that things would be difficult after black jumps to C.]))(;W[ob]C[...later];B[nb];W[pb];B[nc];W[mb];B[na];W[rd];B[re];W[rc]C[White can make a life in the corner like this, but since this also damages white's potential territory at the top, white needs to wait for the right time to do it (not immediately).]))(;B[lf]TR[jj]C[The best option now is to extend. You can see black's tengen stone is getting disconnected, but there's nothing better.]))(;B[gd]LB[hc:A]TR[dd][gf]C[In this case it's good to throw a quick punch while white's off balance. If black doesn't do this immediately, white's move at A would be very big and would also repair the shape.];W[ef]LB[df:A]C[If white repairs the shape, like this or at A...];B[ig]LB[gd:1][ef:2][gf:A]C[Black can go back to the center with a move like this and the 1-2 exchange was good for black. The game still feels well balanced so you can't really criticize A, except to say it's strange..]))(;W[rd]C[...later];B[re];W[rb];B[sd];W[sc]C[Later in the game white can make a ko for life, like this.];B[se]C[If black declines the ko fight...];W[ob];B[nb];W[pa]C[...white just lives outright.]))(;B[ob]C[Black can't completely erase white's aji in the corner, but this move is slightly more severe than the move in the game.];W[rd]C[...later];B[re];W[rb]C[White can still cause some trouble with these moves.]))(;W[ob]C[It would be normal for white to continue like this and...];B[nc];W[nb];B[mc];W[rd];B[re];W[rc];B[qf]LB[jd:A]C[White gets sente. Black's stone at A is too close to black's strong position now, but black's influence is very powerful, so black can still be fairly happy with this result.]))(;B[pn]LB[lm:B][mm:A][qn:C]C[Black might think about playing here instead, because the game is turning into a contest of large frameworks. This move has more of an effect on the center and makes it easier to expand black's framework (while reducing white's too) at A or B later.]))rong position now, but black's influence is very powerful, so black can still be fairly happy with this result.]))(;B[pn]LB[lm:B][mm:A][qn:C]C[Black might think about playing here instead, because the game is turning into a contest of large frameworks. This move has more of an effect on the center and makes it easier to expand black's framework (while reducing white's too) at A or B later.]))