Hi

This is only my second post, so forgive me if I'm going over old ground here.

I have written a solver program (which I'll post up here after I've tidied it up a bit).

It works as follows (I don't know any of the terminology, so bear with me please):

(1) look at each cell, and try to eliminate all but 1 number.

(2) look at each row, column and box, and try to find if each number has only one place to go.

Whenever (1) has covered the entire grid, if it finds any new numbers, it repeats for another pass. If not, it carries on to (2).

Whenever (2) finds something, it calls (1) again, and so on until it solves or gives up.

It is very successful, solving all puzzles in the Irish Times (http://www.ireland.com ), which publishes an easy, medium and hard puzzle every day (not Sundays).

I've keyed in some "hardest ever" puzzles, and it just gives up.

I'm planning to add a "guess" stage next, which I think will enable it to solve any puzzle no matter how hard.

My question is:

Is there any need for a "matching pairs" stage? Can you give me an example of a puzzle that can't be solved by (1) and (2) above, but can by finding matching pairs? Apologies if it's a silly question.