FNLSpring:

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   Nonlinear estimator spring model. Used by CDKFDemo.
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   Form:
   [xDot, fData] = FNLSpring( x, t, fData )
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   ------
   Inputs
   ------
   x	      (1,1)  State
   t         (1,1)  Time
   fData     (1,1)  Data structure
                    .k  (1,1) coefficient
                    .x0 (1,1) nominal x

   -------
   Outputs
   -------
   xDot      (:)    Derivative
   fData     (1,1)  Data structure
                    .k  (1,1) coefficient
                    .x0 (1,1) nominal x

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