Hello.

I am quite new to Sudoku and have been trying to go beyond the basic techniques so that I can have a go at some of the harder puzzles. I understand the X-wing, and even the XY-Wing, but the Swordfish is still confusing me. I read these http://angusj.com/sudoku/hints.php#swordfish & http://www.sadmansoftware.com/sudoku/swordfish.htm

"Look for three columns with only two candidates for a given digit. If these fall on exactly three common rows, and each of those rows has at least two candidate cells, then all three rows can be cleared of that digit - except in the defining cells. This is the original, "restrictive" definition. It has since been realised that a more relaxed definition is possible, in that the three columns can each have two or three candidates for the given digit - as long as they fall on the three common rows."

So I thought I had discovered a Swordfish in a puzzle I was really struggling with, but I ended up with an error and wasnt able to complete. So I was wondering if my Swordfish was correct. I chose columns A B and C, as the defining cells fell on 3 rows. I cancelled the candidates on the ? squares R7>C2 & C5 and R8> C9.

Was this right? Here http://angusj.com/sudoku/hints.php#swordfish it seems to imply that you can remove candidates from rows and columns that lie between defining cells.

If this was right, then maybe I just made a schoolboy error and messed up somewhere else.