HoDoKu {0}

{8}

All the possible positions of the value {1} in {4} base {2} are in the same {4} cover {3}. The number of {2}s matches the number of {3}s, and each {2} must contain the value {1}; so each {3} will have a {1} in one of the intersecting cells.

Hence {1}'s in the covers but not in the bases may be removed.

Therefore we can remove {6}.

Explanation

Mutant Fish use all three types of regions (box, row, and col) as both base AND cover regions. Note that MOST mutant eliminations will have already been found by simpler Fish types, so they don't show-up, and with all three region types in the search the number of combinations of regions EXPLODES, so searching them is MUCH slower; but we find a few, so we do it anyway, mainly just because we can.

Mutant Jellyfish are "too slow to be allowed", as are Kraken Jellyfish, despite my best efforts. Likewise Aligned*Exclusion. sigh.

Note that HoDoKu incorrectly identifies some Mutants as Frankens; but hobiwan's opinion is, in my opinion, superior to my opinion, so please politely forget that I ever mentioned it. Maybe we should change the definitions of Franken and Mutant to align with hobiwans implementation, in his honour. In Sudoku any opinion is as good as any other, and his was a damn sight better than most.

{5}