triple3

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  Fat triplets.  AKA 3 x 3 circles.
  
  Imagine we have:
  
       1         2         3          4         5         6          7         8         9       
  .---------.---------.---------..---------.---------.---------..---------.---------.---------.  
a |         |         |         ||         |         |         ||         |         |         | a
b |         |         |         ||         |         |         ||         |         |         | b
c |         |         |         ||         |         |         ||         |         |         | c
  |---------+---------+---------++---------+---------+---------++---------+---------+---------|  
d |         |         |         || 2 4  7  | 2 4  7  | 2 4  7  ||         |         |         | d
e |         |         |         ||         |         |         ||         |         |         | e
f |         |         |         ||         |         |         ||         |         |         | f
  |---------+---------+---------++---------+---------+---------++---------+---------+---------|  
g |         |         |         ||         |         |         ||         |         |         | g
h |         |         |         ||         |         |         ||         |         |         | h
i |         |         |         ||         |         |         ||         |         |         | i
  '---------'---------'---------''---------'---------'---------''---------'---------'---------'  
       1         2         3          4         5         6          7         8         9       

  How to analyze rigorously?

  (The iff argument gets messy.)


    digits  2 4 7  | possibilities
           ------- |  
    cells   2 4 7  | 2 2 4 4 7 7
            2 4 7  | 4 7 2 7 2 4
            2 4 7  | 7 4 7 2 4 2

  so in all cases all the digits 2,4,7
  are monopolized by d4, d5, and d6.