(;GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2]
RU[Japanese]SZ[19]KM[0.00]
PW[CHH]PB[XXX]
;B[pd]
;W[dp]
;B[pq]
;W[dc]
;B[de]
;W[ce]
;B[cf]
;W[cd]
;B[fq]
;W[cn]
;B[dg]
;W[fc]
;B[ck]
;W[dr]
;B[jq]
;W[qf]LB[dc:4][fc:12][cd:8][pd:1][ce:6][de:5][cf:7][qf:16][dg:11][ck:13][cn:10][dp:2][fq:9][jq:15][pq:3][dr:14]
;B[qh]LB[dc:4][fc:12][cd:8][pd:1][ce:6][de:5][cf:7][qf:16][dg:11][qh:17][ck:13][cn:10][dp:2][fq:9][jq:15][pq:3][dr:14]C[First let's consider why white played 18. What is wrong with the obvious invasion at sansan? ]
(;W[qn]LB[ql:C][po:B][qp:A]C[Starting position - White 3 is a testing move. How should black play so as to avoid falling  into a trap? 

DW - A trap? -it is hard to see white 18 as a trap, it all looks so normal, but to a pro's eyes something looks fishy !   

This time it is just three alternatives A, B, C to choose from but in each case the follow up is important and involves  knowledge of some not too complicated joseki. 

Given as Black you have a knowledge of these joseki the important question is what is it you are trying to achieve in considering this position? What is the position as Black you will be satisfied achieving over the next 20 moves or so?
Lets take a look at a top pro's analysis. (Back to move 17)

]
(;B[qp]C[Variation A1 - Black takes real territory. If White plays the normal move to settle the position then Black is satisfied to suppress the stone at R14.  the following sequence can be envisaged.  ]
(;W[qk]
;B[pf]
;W[pe]C[White would like to live large in the corner but this is not going to happen when black cuts.]
;B[qe]
;W[oe]
;B[qd]
;W[pg]
;B[of]LB[qd:25][oe:24][pe:22][qe:23][of:27][pf:21][pg:26][qk:20][qn:18][qp:19]C[This sequence is very good for Black as the ladder is good. Therefore White has to play at R15 instead of Q15, which results in a low, uncomfortable, position. ])
(;W[of]C[Variation A2 - White will not follow Black's wishes but instead plays actively. ]
;B[nd]C[Black 3 is virtually forced ]
;W[pi]C[White attempts to surround the black stones (DW: This shape comes up a lot in actual games). 

]
;B[ph]
;W[oh]LB[nd:21][of:20][oh:24][ph:23][pi:22][qp:19]
(;B[oi]C[Variation A3  - This black 7 is unreasonable in this position. The result to white 16 is almost hopeless for Black. 
(DW: it is well worth remembering the throw-in of white 10) .

]
;W[qi]
;B[ri]
;W[rh]
;B[rg]
;W[qg]
;B[sh]
;W[pg]
;B[rh]
;W[qk]LB[nd:21][of:20][pg:32][qg:30][rg:29][oh:24][ph:23][rh:33][sh:31][oi:25][pi:22][qi:26][ri:27][qk:34][qn:18][qp:19])
(;B[qi]C[Variation A4 - It is important for Black to play 27 and 29 to link up, but up to 34 White gets a strong outside position and Black is not satisfied. ]
;W[pj]
;B[qj]
;W[pk]
;B[ql]
;W[pl]
;B[qm]
;W[pm]
(;B[rn]
;W[pg]LB[pg:34][qi:25][pj:26][qj:27][pk:28][pl:30][ql:29][pm:32][qm:31][rn:33])
(;B[pn]C[Variation A5: as the previous variation did not work so well, what about if Black cuts? ]
;W[po]
;B[on]
;W[oo]
;B[nn]
;W[qo]LB[nn:37][on:35][pn:33][oo:36][po:34][qo:38]C[After white 34, 36 and 38 White has separated Black into 2 groups. Black is in "an unfortunate position to do battle".

To summarise:  Black A in the initial diagram will turn out badly if White finds the best moves. ]))))
(;B[po]C[Variation B - Black takes real territory. If Black pincers at 21 then the joseki variation from 22-33 follows. The result is White has settled the position in sente and has first move at the top. The continuation up to white 42 demonstrates that White has settled 2 groups succesfully and this is unsatisfactory for Black.   ]
;W[rp]
;B[ql]
;W[pn]
;B[qo]
;W[ro]
;B[qq]
;W[nn]
;B[rm]
;W[rn]
;B[mp]
;W[pl]
;B[pk]
;W[ok]
;B[pj]
;W[qc]
;B[qd]
;W[pc]
;B[od]
;W[rd]
;B[re]
;W[rc]
;B[qe]
;W[nc]LB[nc:42][pc:36][qc:34][rc:40][od:37][qd:35][rd:38][qe:41][re:39][pj:33][ok:32][pk:31][pl:30][ql:21][rm:27][nn:26][pn:22][rn:28][po:19][qo:23][ro:24][mp:29][rp:20][qq:25])
(;B[ql]C[Variation C1 - This black 19 squeezes (pincers) the white stone on the lower side and is the correct answer! 
White attempts to settle quickly. If Black allows this then White will be able to play first at the top, as in the previous diagram. This is unsatisfactory for Black. Also note point A, which is "an open door " and reduces the value for Black of the right side. ]
;W[qq]
(;B[pp]
;W[qp]
;B[po]
;W[rn]
;B[om]
;W[qc]
;B[qd]
;W[pc]
;B[od]
;W[rd]
;B[re]
;W[rc]
;B[qe]
;W[nc]LB[nc:34][pc:28][qc:26][rc:32][od:29][qd:27][rd:30][qe:33][re:31][ql:19][rl:A][om:25][rn:24][po:23][pp:21][qp:22][qq:20])
(;B[qp]C[Variation C2 - This is the correct answer. Black should prevent White settling in sente. Up to 33, Black gets sente and importantly plays 35 to take the initiative at the top. DW: Black avoided the trap! 

DW: so to simplify greatly; in summary, 'avoiding the trap' is for Black to keep sente on the right side to play first at the top and take the initative.]
;W[pp]
;B[rq]
;W[qr]
;B[po]
;W[op]
;B[qo]
;W[rr]
(;B[mp]
;W[no]
;B[pn]
;W[nq]
;B[mq]
;W[nr]
;B[qe]LB[qe:35][ql:19][pn:31][no:30][po:25][qo:27][mp:29][op:26][pp:22][qp:21][mq:33][nq:32][qq:20][rq:23][nr:34][qr:24][rr:28])
(;B[or]C[Variation C3 - In the actual game the moves in the previous diagram followed, but Black tried  a more complicated sequence with black 29. The result: from 30 -48 Black did get sente to play at 49 and take the initiative; however there was no happy ending, as White won the game by half a point after 271 moves!  ]
;W[mq]
;B[mr]
;W[lq]
;B[lr]
;W[oq]
;B[nr]
;W[pr]
;B[mo]
;W[nn]
;B[pn]
;W[mn]
;B[nq]
;W[np]
;B[mp]
;W[no]
;B[lp]
;W[rp]
;B[ro]
;W[sq]
;B[qe]
;W[pf]
;B[nd]LB[dc:4][fc:12][cd:8][nd:51][pd:1][ce:6][de:5][qe:49][cf:7][pf:50][qf:16][dg:11][qh:17][ck:13][ql:19][cn:10][mn:40][nn:38][pn:39][qn:18][mo:37][no:44][po:25][qo:27][ro:47][dp:2][lp:45][mp:43][np:42][op:26][pp:22][qp:21][rp:46][fq:9][jq:15][lq:32][mq:30][nq:41][oq:34][pq:3][qq:20][rq:23][sq:48][dr:14][lr:33][mr:31][nr:35][or:29][pr:36][qr:24][rr:28]))))
(;W[qc]
;B[qd]
;W[pc]
;B[od]
;W[rd]
;B[re]
;W[rc]
;B[qe]
;W[nc]
;B[po]LB[dc:4][fc:12][nc:26][pc:20][qc:18][rc:24][cd:8][od:21][pd:1][qd:19][rd:22][ce:6][de:5][qe:25][re:23][cf:7][qf:16][dg:11][qh:17][ck:13][cn:10][po:27][dp:2][fq:9][jq:15][pq:3][dr:14]C[Black will welcome the invasion at the top. The result presents Black with  two important advantages, 

1) sente to play at Q5, which works well with all the black stones.
2) whites stones end low on the top side in particular the value of F17 is  diminished. 

For these reasons white played 18 as in the game. 

]))
