FXNLSpring:

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   Jacobian of RHSUKF.

   Partial of the right-hand-side with respect to the position.

   Since version 11.
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   Form:
   f = FXNLSpring( x, t, d )
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   Inputs
   ------
   x         (1,1)  Position
   t         (1,1)  Time (not used)
   d         (1,1)  Data structure
                    .w   (1,1) Spring constant

   -------
   Outputs
   -------
   f         (1,1)  dF/dx

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