GenSub:

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   Subtract two matrices where NaN - NaN or inf - inf == 0

   Since version 2.
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   Form:
   c = GenSub( a, b )
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   ------
   Inputs
   ------
   a             (n,n)  Matrix
   b             (n,n)  Matrix

   -------
   Outputs
   -------
   c             (n,n) Difference

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