This post continues with two more Hodoku examples, which illustrate how a human solver finds grouped AIC. As usual, Hobinger attributes them to making just the right premise at just the right place.
In the Hodoku Grouped AIC section, the third example is a group connected XY chain. The suggested starting premise is that 5r7c2 is false, and with that, Hodoku sets off to reach a contradiction.
My contention of this series is that there is no reason to start this way, and nobody should do it. It would be reasonable to notice the “almost remote pair” 56-chain and attempt to extend it.
This could lead to an exploration such as shown here, working from the three ends of the 56-chain, and reversing polarities of the strong links. Along the way, the 7-group paths in the C box might be tried, one of them leading to the ANL of the example.
More likely than this effort, and more consistent with the Sysudoku order of battle, would be . . .
to notice the reach of the two easy Medusa coloring clusters that lie beyond the 3579 fog bank.
The two clusters collide in c2 where red and blue claim the true 6, implying green or orange. Thus
blue => orange.
So if we could not find our way to the grouped ANL, we could try blue and orange. With the blue 6 removing the middle 7 in the N box, the orange 7 forces two 7’s in c6. So green wins, soon forcing red.
With the collapse, we can leave satisfied, but wondering how Hobinger’s Hodoku comes up with these examples.
Unlike the other examples I worked through, Hodoku’s last Grouped AIC example did not come straight out of line marking. First you have to apply this 7-wing.
Now on the example grid, Hodoku claims to start with the premise of r2c5 <>4. That’s an arbitrary guess.
The forcing chain looking South makes 1r1c4 true, which makes 4r2c5 true, a Hodoku contradiction.
But get real. On the 1- panel you have a very noticeable grouped chain.
If you didn’t find the i471-wing earlier, you would probably be tracing the 14 bv connecting to the short 4-chain.
We rest our case. Hodoku’s starting premise is actually what a computer does t
housands of times to find a particular instance of a successful technique. It is not a practical or pleasurable way for a human solver to start looking for an AIC.
Oh yes, the i471-wing? It collapses the example puzzle well before AIC in the SOOB. Hodoku didn’t catch it because the solver just doesn’t do “seeing” by forcing chain. Nor do most solvers, for that matter. With the XYZ map, sysudokies do.
Next week, I begin sifting through the very advanced fishing theory of Hodoku’s day, hoping to refine my posts of April/May 2012 on that topic.