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A Mutant Swordfish is a Mutant Fish of size 3.
Here is an example:
All the candidates in rows 1 and 9 and column 7 (the defining set) are located in column 2 and boxes 3 and 9 (the secondary set). To complete these 3 defining constraints, every secondary constraint will contain an instance of the selected digit, allowing us to eliminate all candidates that do not belong to the defining set.
The following example shows that the defining set can also include a box:
In this example, the defining set is row 8, column 8 and box 1. The secondary set is row 2, column 2 and box 9.