The following position arises after a variation of a relatively well known Japanese joseki. The marked black stone is to my knowledge a simplifying play that finishes the position temporarily.

I have two questions about the position: 1) What potential exists for white should he or she wish to play at a? 2) If black returns to the position in sente and takes b or c, how might white handle him or herself locally?

Click Here To Show Diagram Code

[go]$$c This is a label for the diagram.
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , c . . |
$$ | . . . . . . . . . . . . . . . . b . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . O . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . X , O . . |
$$ | . . . . . . . . . . . . O . . a X B . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

Is this really joseki? It looks terrible for White imo.

Click Here To Show Diagram Code

[go]$$Wc This is a label for the diagram.
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . 9 . . |
$$ | . . . . . . . . . . . . . . . . 5 . . |
$$ | . . . . . . . . . . . . . . . 8 . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . a . 2 6 1 7 . |
$$ | . . . . . . . . . . . . 3 0 . . 4 b . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

This is a normal joseki I believe ("a" and "b" after ), so Black has opted for something different but it must be joseki for White.

A commented pro game from my study group

[sgf-full](;GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2]
RU[Japanese]SZ[19]KM[6.50]
PW[Xie Yimin]PB[Mukai Chiaki]BR[4d]DT[2011-03-02]EV[23rd Female Meijin title match]RO[1]SO[Mr. Kin]RE[B+Resign]
;B[qd]
;W[pp]
;B[dc]
;W[cp]
;B[ep]
;W[gq]
;B[cq]
;W[cm]C[topazg: All the cool kids seem to like playing this these days. Is this just more popular than classic variations like D3, or are there specific reasons to choose it here?

Shaddy: Maybe they consider the standard result too thick for black? Or it could just be a trend, like magicwand always says.

Ethan: Maybe if black's marked stone were at C16 white would want to play more thickly with a hane at B3?]
;B[dp]
;W[bp]
;B[dn]
;W[cl]C[topazg: Cute.

Shaddy: I like it, pretty thick.]
;B[fq]
;W[gp]
;B[bq]
;W[ci]C[Shaddy: White's played on both sides, black has the corner and some rather impotent influence.. Seems even, but more even for white than the other way (hehe)

topazg: True, it is a nice corner and sente though

Ethan: and there's the attack coming up. ]
;B[jq]
;W[mq]
;B[jo]
;W[gm]
;B[mo]
;W[oq]C[topazg: I found this one interesting. No attempt to do anything other than secure the corner in the most solid way possible. I'd have been worring about G4 :S

Shaddy: Well, it's pretty sente for black. She can settle her group pretty easily if White omits this: gotta be thick if you want to attack.

topazg: I agree, it's just the patience of this move that strikes me. I try to do things too sente-ish-ly]
;B[ho]
;W[go]
;B[gn]
;W[hn]
;B[fn]
;W[io]
;B[hp]
;W[ip]
;B[hq]
;W[iq]
;B[hr]
;W[ir]
;B[in]
;W[hm]
;B[gr]
;W[jp]
;B[im]
;W[ko]C[topazg: An interesting way to sacrifice. I'd have played K6 instead of J2. I really underestimate edge plays I think.

Shaddy: It'd be intolerable to me to allow black the connection underneath, esp. because w would be looking pretty weak.

topazg: I totally agree, I just know I wouldn't have done it]
;B[il]
;W[qm]
;B[fl]C[topazg: I found this really interesting too. I would have considered the big corner plenty, and the J7 stones "capable of handling themselves", and tried for a big fuseki point, probably in one of the top corners. The aji of White peeping at G9 and H9 I would have thought made this not good enough to play. Professionals seem to excel at good timing of "solid" moves.

Shaddy: Seems like playing an extension when your third-line stone is approached- sure, if you get pincered you can run, but you'd rather not have to.

topazg: J6+J7+J8+F8 seems a lot of stones to exchange for the value (to my eyes) of K4, L5, and R7, that's all

Shaddy: This is white's side of the board, so it seems natural to me that he should get more here than black.]
;W[od]
;B[ld]
;W[qe]
;B[re]
;W[pe]
;B[rf]
;W[qc]
;B[rc]
;W[pd]
;B[rd]
;W[qb]
;B[qi]C[topazg: This is another common modern joseki. Again, something I just wouldn't do from a high far pincer - is this just the "cool" thing right now, or is there a reason this is superior in some cases to either P14 or Q13 (my normal choices)?

Shaddy: I always hated this joseki for both sides. No substantive comment.]
;W[mc]C[topazg: We all know these sorts of shoulder hits are bad right? Interesting that this way of settling is so important for White right now - also interesting to me is the hane and connect at M18. Presumably this is playable because Black doesn't have a stone around C14 or C13?

Shaddy: Yeah, w will have the clamp aji if black encloses and she gets to approach if black protects there. Neat miai.

topazg: Agreed.]
;B[lc]
;W[lb]
;B[kb]
;W[mb]
;B[cg]C[Shaddy: This is a funny enclosure. Wonder why?

topazg: I'm guessing B10 aji now ]
;W[hs]C[Shaddy: Timesuji, or something more?]
;B[fr]
;W[jc]C[topazg: This feels like overplay. I'll remember it isn't!

Shaddy: This stupid thing is always so uncomfortable.]
;B[kc]
;W[jb]
;B[jd]
;W[id]
;B[je]
;W[hc]
;B[ja]
;W[ia]
;B[ka]
;W[hb]C[topazg: In hindsight, Black doesn't look very cheerful.

Shaddy: This is a move I wouldn't play, I'd probably do h14 instead. It's definitely better than my move though.. White looks really hard to attack, and the corner is looking thinner

topazg: Agreed, I'll remember this sequence - it can't be _that_ rare.]
;B[md]
;W[rb]C[topazg: Originally, I figured this was just to stop Black getting S18 in sente after wO16 (as presumably bP18 as a follow-up is severe). In hindsight, Black's group on the right isn't that happy.

Shaddy: It looks like a really thick group 'cause it's got all that second and third line stuff, right? But last time I had this thing as black, I got killed.. This is part of why I hate this joseki.]
;B[fb]
;W[ie]
;B[if]
;W[hf]
;B[nc]
;W[qg]
;B[rg]
;W[nb]
;B[nd]
;W[oc]
;B[hg]
;W[ge]
;B[gf]
;W[he]
;B[ig]
;W[qh]
;B[rh]
;W[rj]
;B[ri]C[topazg: Black is reduced to one eye on the edge

Shaddy: It's one and a half

topazg: Sorry, yeah, I meant that.]
;W[ff]
;B[bj]
;W[bi]
;B[cj]
;W[dj]
;B[di]
;W[ej]C[topazg: Very surprising! Why give up two stones on the edge with such a weak group in the balance?]
;B[ch]
;W[el]
;B[gg]
;W[fk]
;B[gl]
;W[gk]
;B[hl]
;W[qk]C[topazg: Black can make two eyes in gote on the edge, but it's becoming to think about I would guess. Is this White's way of stopping Black from building territory around M13?]
;B[mm]
;W[de]
;B[dh]C[topazg: Again, I'm clueless. I'd have played C15 like a shot. None of the ladders work if White cuts at D12. Is this group really in trouble of not being able to get two eyes, so Black reinforces locally instead of jeapordising the top left?

Shaddy: Seems like it'd be pretty painful for black to allow w to connect everything in sente. She'd be attacked everywhere..

topazg: Makes sense, the cut doesn't work to rescue B11, but that wouldn't be the point of it.]
;W[cc]
;B[bl]
;W[bm]
;B[bk]
;W[ap]
;B[dm]
;W[dl]
;B[ee]TR[ed][fe]
(;W[fe]C[topazg: I'd have played E16. Why here?]
;B[ef]
;W[db]
;B[fj]
;W[ei]
;B[fi]
;W[eh]
;B[fg]
;W[eg]
;B[df]
;W[hj]
;B[ii]
;W[ij]
;B[ji]
;W[jj]
;B[kj]
;W[fh]TR[gi][lp]
(;B[lp]C[topazg: Does G11 not work now ? I can't see White's second eye (E9 and C9 are miai to remove it)]
;W[ki]
;B[kh]
;W[li]
;B[lj]
;W[lh]
;B[kg]
;W[lg]
;B[kf]
;W[hi]C[topazg: M4 is a guaranteed cash gain after M3, but White has done a lot of work in the middle. I'd love to know how White was supposed to escape if Black had extended earlier.]
;B[lq]
;W[mj]
;B[ll]
;W[ed]
;B[ce]
;W[sc]
;B[oh]
;W[og]
;B[ng]
;W[nh]
;B[pg]
;W[lk]
;B[kk]
;W[mk]
;B[of]
;W[lr]
;B[kr]
;W[mr]
;B[dd]
;W[kl]
;B[jn]
;W[kp]
;B[fd]
;W[ln]
;B[gd]
;W[eb]
;B[ec]
;W[bd]
;B[lm]
;W[mn]
;B[nm]
;W[jk]
;B[ol]
;W[pj]
;B[km]
;W[nn]
;B[on]
;W[pn]C[topazg: Pro trades leave me speechless somehow.

Shaddy: That was ridiculous.]
;B[ea]
;W[bb]
;B[da]
;W[cb]
;B[oo]
;W[no]
;B[po]
;W[qo]
;B[qp]
(;W[pq]C[topazg: Interesting choice of connection. Was a blind spot to me, but Black mustn't get P4 in sente, so it sort of makes sense.

Shaddy: Why not just atari at p4 yourself? makes no sense to me

topazg: Can't Black live though?

Shaddy: Derp, missed that.]
;B[ro]
;W[qn]
;B[rl]
;W[ql]
;B[qq]
;W[rn]
;B[rq]
;W[ps]C[Shaddy: hehe, nice. I always forget this one]
;B[kn]
;W[lo]
;B[qr]
;W[sr]
;B[sj]
;W[rk]
;B[sk]
;W[si]
;B[sh]
;W[pi]
;B[qf]
;W[ph]
;B[pf]
;W[mf]
;B[ni]
;W[mh]
;B[sd]
;W[ne]
;B[sf]
;W[aq]
;B[oe]
;W[ob]
;B[nj]
;W[oi]
;B[ok]
;W[og]C[topazg: I presume this ko fight is as big as it looks - is Black not dead too quickly in the lower right for White to be worried about this?

Shaddy: White would rather not be forced to fill the liberties there.

topazg: Ah, of course, that's 6-7 points at least.]
;B[co]
;W[bo]
;B[oh]
;W[em]
;B[fm]
;W[og]
;B[pk]
;W[qj]
;B[oh]
;W[op]
;B[om]
;W[og]
;B[is]
;W[js]
;B[oh]
;W[br]
;B[cr]
;W[og]
;B[bs]
;W[ag]
;B[bh]
;W[ca]
;B[sl]
;W[sn]
;B[fa]
;W[oh]
;B[ar]
;W[cn]
;B[jl]
;W[kk]
;B[be]
;W[ae]
;B[gh]
;W[gi]
;B[sb]
;W[do]
;B[en]
;W[ih]
;B[jh]
;W[hh]
;B[lf]
;W[nf]
;B[me]
;W[mg]
;B[bf]
;W[af]
;B[al]
;W[ah]
;B[ai]
;W[cd]TR[ac][gs]C[Ethan: Black's cutting stones can escape, but it
looks like she loses the capturing race above.]
(;B[gs])
(;B[ac]
;W[ad]
;B[ab]
;W[aa]
;B[ba]C[Ethan: I guess since she's a few points behind anyway she'd welcome a ko.]))
(;W[op]
;B[ro]
;W[om]
;B[qq]C[topazg: Alive?]
;W[pr]
(;B[qr]
;W[rn]
;B[so]C[Shaddy: Ko, but white can't afford this I guess.
Then again, I can't count ko threats and my count has white behind by more than a few points. Perhaps w should just fight the ko out, esp. because of the 6 extra pts from the capture and the potential for o8 later..])
(;B[rr]
;W[rn]
(;B[rp]
;W[sq]
;B[rs]
;W[sr])
(;B[sp]
;W[rs]
;B[qs]
;W[qr]
;B[ss]
;W[sq])
(;B[so]
;W[sq])
(;B[qs]
;W[rp]))))
(;B[gi]
;W[hi]
;B[gh]
;W[kk]TR[lj][jk]
(;B[lj]
;W[jl]
;B[km]
;W[lk]
;B[mk]TR[ki][mj]
(;W[ki])
(;W[mj]TR[mi][nl]
(;B[mi]
;W[ml]
;B[nk]
;W[lm]
;B[nl]
;W[ll]
;B[ln]
;W[nm]
;B[mn]
;W[kn]TR[mm][ln][mn][mo]C[Ethan: Trouble.])
(;B[nl]C[Ethan: Nice defense on this side, but problems the other way.]
;W[ki]
;B[li]
;W[kh]
;B[jg]
;W[lh]
;B[mi]
;W[nj]
;B[mh]
;W[nk]
;B[ml]
;W[mg]
;B[nh]
;W[ng]
;B[oh]
;W[kf]
;B[kg]
;W[lg]
;B[lf]
;W[pi]
;B[pj]
;W[oi]
;B[oj]
;W[ok]C[Ethan: Black's cutting stones can escape, but it looks like she loses the capturing race above.])))
(;B[jk]
;W[ki]TR[jf][kh]
;B[lj]
(;W[jf]
;B[kg]C[topazg: Is Black ok here?])
(;W[kh]
;B[jg]
;W[kg]
;B[kf]
;W[mg]C[Shaddy: Black doesn't seem ok here.

(or whatever, I'm not sure where 150 should be. But it seems black gets sealed in and I think he's short liberties.)]))))
(;W[ed]
;B[dd]LB[fd:B][ce:A]C[emerus: my guess]
;W[cd]C[Ethan: I dunno, this one is hard to judge.]
;B[df]
;W[ce]
;B[fd]
;W[ec]
;B[fc]
;W[db]
;B[fe]
;W[gd]
;B[fj]))[/sgf-full]

You are of course right that the main move is 6 for black. Pros presumably don't play the marked black stone because they feel they can do better. The move was apparently suggested by Guo Juan as a simple way to settle the position (check Josekipedia for reference.)