improve class Sudoku9x9Ext to track possible
--- index ---
 1  2  3  4  5  6  7  8  9 
10 11 12 13 14 15 16 17 18 
19 20 21 22 23 24 25 26 27 
28 29 30 31 32 33 34 35 36 
37 38 39 40 41 42 43 44 45 
46 47 48 49 50 51 52 53 54 
55 56 57 58 59 60 61 62 63 
64 65 66 67 68 69 70 71 72 
73 74 75 76 77 78 79 80 81 
--- value ---
 5  3  .  .  7  .  .  .  . 
 6  .  .  1  9  5  .  .  . 
 .  9  8  .  .  .  .  6  . 
 8  .  .  .  6  .  .  .  3 
 4  .  .  8  .  3  .  .  1 
 7  .  .  .  2  .  .  .  6 
 .  6  .  .  .  .  2  8  . 
 .  .  .  4  1  9  .  .  5 
 .  .  .  .  8  .  .  7  9 
--- Sudoku9x9Ext.toPrint()---
   1 5 possible{} 
   2 3 possible{} 
   3 0 possible{123456789} 
   4 0 possible{123456789} 
   5 7 possible{} 
   6 0 possible{123456789} 
   7 0 possible{123456789} 
   8 0 possible{123456789} 
   9 0 possible{123456789} 
  10 6 possible{} 
  11 0 possible{123456789} 
  12 0 possible{123456789} 
  13 1 possible{} 
  14 9 possible{} 
  15 5 possible{} 
  16 0 possible{123456789} 
  17 0 possible{123456789} 
  18 0 possible{123456789} 
  19 0 possible{123456789} 
  20 9 possible{} 
  21 8 possible{} 
  22 0 possible{123456789} 
  23 0 possible{123456789} 
  24 0 possible{123456789} 
  25 0 possible{123456789} 
  26 6 possible{} 
  27 0 possible{123456789} 
  28 8 possible{} 
  29 0 possible{123456789} 
  30 0 possible{123456789} 
  31 0 possible{123456789} 
  32 6 possible{} 
  33 0 possible{123456789} 
  34 0 possible{123456789} 
  35 0 possible{123456789} 
  36 3 possible{} 
  37 4 possible{} 
  38 0 possible{123456789} 
  39 0 possible{123456789} 
  40 8 possible{} 
  41 0 possible{123456789} 
  42 3 possible{} 
  43 0 possible{123456789} 
  44 0 possible{123456789} 
  45 1 possible{} 
  46 7 possible{} 
  47 0 possible{123456789} 
  48 0 possible{123456789} 
  49 0 possible{123456789} 
  50 2 possible{} 
  51 0 possible{123456789} 
  52 0 possible{123456789} 
  53 0 possible{123456789} 
  54 6 possible{} 
  55 0 possible{123456789} 
  56 6 possible{} 
  57 0 possible{123456789} 
  58 0 possible{123456789} 
  59 0 possible{123456789} 
  60 0 possible{123456789} 
  61 2 possible{} 
  62 8 possible{} 
  63 0 possible{123456789} 
  64 0 possible{123456789} 
  65 0 possible{123456789} 
  66 0 possible{123456789} 
  67 4 possible{} 
  68 1 possible{} 
  69 9 possible{} 
  70 0 possible{123456789} 
  71 0 possible{123456789} 
  72 5 possible{} 
  73 0 possible{123456789} 
  74 0 possible{123456789} 
  75 0 possible{123456789} 
  76 0 possible{123456789} 
  77 8 possible{} 
  78 0 possible{123456789} 
  79 0 possible{123456789} 
  80 7 possible{} 
  81 9 possible{}