The New Sudoku Players' Forum

[SpAce]

Nevertheless, I see a nuance between the present case and previous examples. In previous examples, one guardian was discarded because demonstrated (with a chain) to have the same parity as an other one.
In the current BUG+5, choosing the set of four guardians #7r2c7, #+7r4c6, #+8r5c5, +8r4c3 is not "reducing the set of guardians" The double symbol #+ in front of 7r4c6 & 8r5c5 draws attention on their role of double guardians. Posting such a solution could be accompanied of a sentence about the comprehensive pattern covering by the set of guardians.

That's pretty cool! A whole new class of double-roled guardians! Took me a while to see what you meant, but I like it. It's pretty easy to see that if either 7r4c6 or 8r5c5 is true then so is 4r4c4, and if neither is true then neither is 4r4c4, and vice versa (i.e. they're equivalent).

To keep the different classes of guardians to a minimum in this case, we could just as well use the original +4r1c7 instead of #7r2c7 because they have no difference in chain length/complexity. Overall that might be the simplest set of guardians with two internals (+4r1c7,+8r4c3) and two doubles (#+7r4c6,#+8r5c5).

[Cenoman]

Double-roled guardians are not so rare. They may be seen notably in L-shaped DP. I used one recently here

[SpAce]

Double-roled guardians are not so rare. They may be seen notably in L-shaped DP. I used one recently here

You mean the 2r3c2 being an external to both r3 and c2? That's cool too, but it's still an external only. Much less weird than being both internal and external at the same time! That's a feat usually reserved for optical illusions