RHSBilliards:

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   Billiard ball model.

   2D double integrators. t is not used.
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   Form:
   xDot = RHSBilliards( x, t, d )
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   Inputs
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   x	 (4*nBalls,1)	    State [x;y;vX;vY]
   t    (1,1)          Unused.
   d    (1,1)          Data structure
                        .nBalls (1,1) Number of balls

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   Outputs
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   xDot (4*nBalls,1)	  State d[x;y;vX;vY]/dt

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