HNLSpring:

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   Nonlinear estimator measurement spring model
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   Form:
   [y, h] = HNLSpring( x, t, hData, k )
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   ------
   Inputs
   ------
   x	      (2,1)  State
   t         (1,1)  Time
   hData     (1,1)  Data structure
                    .alpha (1,1) coefficient
   k         (1,1)  Not used

   -------
   Outputs
   -------
   y         (:)    Output
   h         (1,1)  Derivative of y with respect to x

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