Thanks very much Mike for your helpful explanation. It is more complicated than I thought. This reminded me of another example of what looks an XY-X chain (SS actually called it an "XY Chain").
From A170 v1.5, Afmob's optimized WT (
here) step 3h with all the 9's highlighted:
4. XY-X Chain [edit on name following udosuk]-> no 7 in r4c7. Like this.
4a. If r3c7 is not 7 it's 8. When it's 8 -> r3c3 = 9 -> r4c4 = 8 -> 9 in r4 must be in r4c7 -> no 7 in r4c7
4b. If r3c7 = 7 -> no 7 in r4c7
ALT ways of expressing it courtesy of udosuk:
"r4c47 form a strong link of 9 on r4.
Therefore r34c7=[87] would force r3c3+r4c4=[99], contradiction.
Hence r4c7 can't be 7."
"(Or alternatively, one of r3c3+r4c4 must be 8, forcing r3c7=7 or r4c7=9, so r4c7<>7 no matter what.)"
I quite like it.
Cheers
Ed