This post encounters what seems to be a monster in Castillo’s Only Extreme 304. A stingy basic leaves a near monster cloud of candidates resistant to the ordinary advanced repertoire. Following a theme of this review, a small coloring cluster is extended across the grid by not-both trials. It takes two more trials in the next post to defeat 304. Or perhaps you can find a less extreme path.
The ordinary looking basic trace
leaves this formidable grid to navigate:
There is a Type 1 UR and a resulting box/line to start, but then I find nothing else.
In the swath of bv cells across the middle, I latch onto a tight little XY-chain intersecting a very modest coloring.
The potential for coloring the XY-chain is enhanced by an AIC with numbers common to both the cluster and the xy chain.
If blue 8r4c2 and 8r3c3 are not both true, then 8r3c3 is green, and working back 7r3c3 is blue, 7r2c2 is green, and 2r2c2 is blue.
Testing terminal 8’s on the subchain from r4c2 is a start. The 8’s are not both true, along with blue, because this forces both 3 and 8 in r1c7.
The cluster expansion removes four candidates. Clearly, a further expansion to 3r3c1 does further damage.
Again the not-both trial, testing blue + 3r3c1, is required for the extension.
If both terminals of the X-chain are true, c6 gets no 3’s. Not acceptable.
The expansion turns 8r3c1 blue, generating three traps that expand the XY-chain further.
The final not-both trial to extend coloring over the XY-chain determines that 3r3c7 and 4r6c7 are not both true. If they are, 1,3, and 7 are removed from r2c5, and 1 and 3 are removed from r9c4, leaving two 4’s in c5.
Unless you have found a less extreme path, let’s see how you would continue with the fully colored XY-chain. My finish of Only Extreme 304 comes next week.