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   A linear measurement.
   if d is not entered y = x. Otherwise y = x(d)
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   Form:
   y = GXUKF( x, d )
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   ------
   Inputs
   ------
   x         (:,1)  Input
   d         (1,1)  Indexes to states

   -------
   Outputs
   -------
   y         (:,1)  Output

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   References: None
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