In the Dave Green 5-star of 1/23/22, four 4-fills are available before starting the bypass values list, on r2, r8, c2, and c8. It brings up the possibility that 4-fills can solve this Conceptis puzzle by themselves? We normally put 4-fills first, but in this unusual case, we’ll try to resolve each new 4-fill as clues and subsets are placed.
The grid after the first four has two of the starting 4-fills resolved.
The trace shows that NW4 from the resolution of the 4th, reduces the 3rd to a 3-fill, which is immediately resolved.
The fill resolution style collapse is paused only to show how in tracing, effects of the same value as the cause are listed first. For example, here C2 is the newest cause, and the effect list will read (N2, E2, N6). The reason is, that, in posting new effects in PowerPoint, it’s easier to use all the copies of one value before changing to another value.
That effect ordering appears in the final trace, which shows how the 4-fills go all the way.
The box names and values makes it easy, doesn’t it? Now imagine how easy it is to trace down what went wrong, when you find that you departed from the solution on a particular placement. Human engineering
The next Dave Green surprise is a Saturday 4-star that denies the bypass a single clue. You might think that deserves more stars, but it turns that 4 is enough. The post will demonstrate an alternate route to an easier bypass, when a Sudoku composer like Dave pulls this on you.
Systematic Sudoku 