The X-wing is the easiest fish to spot. So easy, that the process of line marking finds it if you’re paying attention. KrazyDad’s Super-Tough v.5, b.3, #5 demonstrates.
The last line marked was r1, because it had the most free cells. When two 9 candidates line up with two already marked in r3, the new 9-wing rips through the bv.
In the basic trace, the bypass starts with a naked pair and resolves two 3-fills. It’s a good one to read through. Box marking reserves three cells. The 9-wing removal triggers a steep collapse, before we close the columns.
Next week, it’s KrazyDad Super-Tough v. 5, b.4, #5, on krazydad.com.
Systematic Sudoku 