RHSAutomobileXY:

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   Double integrator model for a car.
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   Form:
   xDot = RHSAutomobileXY( x, t, d )
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   ------
   Inputs
   ------
   x	(4,1)   State [x;y;vX;vY]
   t         Time, unused
   d         Data, unused

   ------
   Outputs
   -------
   x	(4,1)   State derivative d[x;y;vX;vY]/dt

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