triple1

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  We can generalize from twins to triples.
  
  Skinny triples.  AKA 2 x 3 circles.
  
  Imagine we have:
  
       1         2         3          4         5         6          7         8         9       
  .---------.---------.---------..---------.---------.---------..---------.---------.---------.  
a |         |         |         ||         |         |         ||         |         |         | a
b |         |         |         ||         |         |         ||         |         |         | b
c |         |         |         ||         |         |         ||         |         |         | c
  |---------+---------+---------++---------+---------+---------++---------+---------+---------|  
d |         |         |         || 2 4     |   4  7  | 2    7  ||         |         |         | d
e |         |         |         ||         |         |         ||         |         |         | e
f |         |         |         ||         |         |         ||         |         |         | f
  |---------+---------+---------++---------+---------+---------++---------+---------+---------|  
g |         |         |         ||         |         |         ||         |         |         | g
h |         |         |         ||         |         |         ||         |         |         | h
i |         |         |         ||         |         |         ||         |         |         | i
  '---------'---------'---------''---------'---------'---------''---------'---------'---------'  
       1         2         3          4         5         6          7         8         9       

  d4 = 2 iff d6 = 7 iff d5 = 4.
  d4 = 4 iff d5 = 7 iff d6 = 2.

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