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  d8 = 2 => d2 = 3 => e1 = 2 => g1 = 4 => g8 = 2 => 
  d8 = 3; this is a contradiction.

  So, is this a deduction or a guess?

  I say it has elements of both.
  
    With a guess you plug in a value and let
      the program pursue an ever expanding tree
      of consequences until a contradiction (or a
      success or a stall) arises.  Here we followed 
      only a single chain of deductions so it is not
      like a guess.
      
    OTOH, with a deduction you look for a known 
      pattern to which you can apply known rules
      to achieve a known result.  Here we pursued
      a pattern we had never seen before so it 
      was not like a deduction.
  
  This example shows how blurry the line is
  
    between deducing and guessing.
    

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