A square where 3-in-2 crosses 4-in-2 must be 1, which is just a combination of your technique for listing the possibilities for rows and columns.

Once in a while I get a row that has two squares that can be either 1 or 2 - something like (12345) (12) (1234) (1235) (12). Then none of the rest of the row can be either 1 or 2, so I can simplify to (345) (12) (34) (35) (12).

If a row has two elements, I can often eliminate a few possibilities by just subtracting from the sum. For example if I have (23456) and (anything) that must sum to 10, I can simplify to (23456) (45678). And 5-5 is not allowed, so it simplifies further to (2346) (4678). If I later eliminate 8 from the second square, I can eliminate 2 from the first square.

Also, if you have an 7-in-2, neither one can be 7, 8, or 9. Similarly, if you have 10-in-3, none of the three can be 8 or 9.

- Hollywood