Sudoku Tutorial
Finned Fish (Finned X wing)
This is almost an X wing.  If the blue cell did not contain candidate 1, then an X wing would be present.  The slashed 1's could be removed (as well as two other cells)..  

Unfortunately this is not the case.  The blue cell contains a 1.   The blue cell is the fin and it spoils our X wing.
Consider just two possibilites:
  1. If the blue fin is a 1,  this would eliminate the slashed 1's in box 2.
  2. If the blue fin is not a 1, we would have a nice X wing which would also eliminate the slashed 1's.
In either case, we can eliminate the slashed 1's in box 2

Note:  we cannot eliminate the 1's in r8c5 or the 1 in r3c3.  That's because this isn't
an X wing.    If this was a true X wing then those candidate could be eliminated.  The fin
confines our possible eliminations to the box where the fin resides (box 2 in this case)
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