squares4

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  These four cells monopolize the values of 8
  in columns 2 and 8.
  
  Therefore, we can get rid of the other 8's
  in those columns.
  
not c2 g2 c8 d8 g8 8

       1         2         3          4         5         6          7         8         9       
  .---------.---------.---------..---------.---------.---------..---------.---------.---------.  
a |         |    5  8 |         ||         |         |         ||         |    5  8 |         | a
b |     6  9|         |1   5    ||   45   9|         |         ||1  4 6   |         |1   5    | b
c | 2   6  9|    5  89|12  5  8 ||         |   45    |   45   9||1 34 6 8 |   456 8 |1 3 5  8 | c
  |---------+---------+---------++---------+---------+---------++---------+---------+---------|  
d |         |         |         ||    5  89|         |1   5  89||1      8 |    5  89|         | d
e |         |         |         || 2     8 |         |         || 2     8 |         |         | e
f |         |         |         || 2 45   9|1  45    |1  45   9||12       |         |1   5   9| f
  |---------+---------+---------++---------+---------+---------++---------+---------+---------|  
g | 2    7 9|   45  89| 2  5  8 ||   45  8 |1  45    |1  45  8 ||  34 678 |   4 6 89|  3    89| g
h |      7 9|         |    5  8 ||         |         |   45  8 ||   4  78 |         |       89| h
i |         |   4   8 |         ||         |         |         ||         |   4   8 |         | i
  '---------'---------'---------''---------'---------'---------''---------'---------'---------'  
       1         2         3          4         5         6          7         8         9