Column squares
Some deductions are based on a square pattern of cells.
Look at b1, b9, h1, h9. Each of the cells has a 9 in it.
Call this a "column" square, because the columns
have only two cells with the value 9.
b1 = 9 iff h1 != 9 iff h9 = 9 iff h1 != 9.
So on line b either b1 or b9 is a 9, and
on line h either h1 or h9 is a 9.
Therefore, we can knock off all the other 9's
on lines b and h.
not b3 b4 b7 h3 h7 9
1 2 3 4 5 6 7 8 9
.---------.---------.---------..---------.---------.---------..---------.---------.---------.
a | | 2 6 | 2 4 6 9|| | |12 6 ||1 4 9|1 9| | a
b | 6 9| | 567 9||1 6 9| | ||1 5 7 9| | 5 9| b
c | | 2 5 7 | 2 45 7 9|| 2 9| 3 9| 23 || 45 789| 5 89| | c
|---------+---------+---------++---------+---------+---------++---------+---------+---------|
d | | |1 56 ||12 6 9| |12 56 || 2 56 9| 56 9| | d
e | | | 56 || | | 2 56 || 2 56 | | | e
f | |1 56 | ||1 6 9| 3 9|1 3 56 || 56 9| | | f
|---------+---------+---------++---------+---------+---------++---------+---------+---------|
g | | 2 6 | 23 6 9|| | | || 3 6 9| 6 9| | g
h | 6 9| |1 6 9|| | | ||1 56 9| | 5 9| h
i | |1 7 |1 3 7 || | | ||1 3 8 |1 8 | | i
'---------'---------'---------''---------'---------'---------''---------'---------'---------'
1 2 3 4 5 6 7 8 9