## Beeby’s Elbow Quad in shye’s Catherine Wheel

This post carries us through the Sysudoku process, with the elbow fireworks of the Catherine Wheel, a second 4-set quad example from shye’s forum posts, cited in Beeby’s notes.

From the givens grill at the end of last week’s post, there are no dublex or cross-hatch clues or subsets for the basic trace. Just a routine box marking, and an uneventful  line marking.

From the line marked grid,

here are the panels showing the elbows of the CW 4-set quad.

Matched elbows on values 1 and 2 on one corner, and another pair on values 4 and 5 on the opposite corner make up a 4-set hidden quad.

As shown last week and earlier for the shye’s other 4-set quad, the removals of 3, 6, 8 and 9 candidates are because the 4 corner cells must solve to the four elbow values.

The 1, 2, 4 and 5 removals are because, in the 4-set, only one of a pair’s two values can be placed in its neck, and placement of the other pair value in the cell opposite its neck makes that value unavailable for either one of its cuffs. An elbow’s value must be placed in one of its three cells. For each elbow pair, one value goes in the neck, and one in a cuff.

In Beeby’s fireworks action, the 4-set quad removals leave clues (NW3, SE3, NE6, SW8), a boxline NEc8 in 1, a naked quad in NE, and NE2 alone in r2.

After this near collapse,

there is a rare opportunity for remote pairs. In a remote pair chain of even length, a candidate of each value is true in one of the end cells. So seeing the end candidates is toxic.

Now the bv field tells me to get out the crayons. The blue/green trap on 6r6c4 creates a naked pair 37 that wipes out a blue 7 and the blue army with it

I just let the green army spread into the solution.

Don’t quite get follow up traces? Just do your own spread of the green from the blue wrap and watch the trace below doing it with you.

Beeby has its own way of finishing the Catherine Wheel. Instead of the remote pairs, it does finned X wings on 6, 7, 8 and 9, then a 6-wing and that naked pair.

For a change of scenery and to make basic examples available for reference in The Sysudoku Guide, my next posts will feature several Dave Green Conceptis puzzles, 5-star and below, that illustrate basic solving resources in the three stages of Sysudoku Basic.

The first puzzle of the series has significant events in all three stages of Sysudoku Basic, the bypass, box marking and line marking. It collapses in the closing stage of line marking, just prior to advanced methods. Try it your way, then read along, following the trace move by move on your grid. You may discover something missing from your Sudoku practice.