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   Nonlinear spring fault. 
   Generates a cubic spring force that grows
   with the function (1 - exp(d.a*dT)) where dT is the time since the
   fault. Prior to the fault, the force is zero.
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   Form:
   f = NonlinearSpringFault( x, t, d )
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   Inputs
   ------
   x		(2,1)  State
   t       (1,1)  Time
   d       (1,1)  Input data structure

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   Outputs
   -------
   f    	(1,1)  Force due to fault

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