BilliardCollision:

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   Billiard collision model.

   Models wall collisions.

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   Form:
   x = BilliardCollision( x, d )
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   Inputs
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   x	 (4*nBalls,1)	State [x;vX;y;vY]
   t    (1,1)          Unused.
   d    (1,1)          Data structure
                       .nBalls (1,1) Number of balls
                       .xLim   (1,2) [xMin xMax]
                       .yLim   (1,2) [yMin yMax]
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   Outputs
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   x	 (4*nBalls,1)	State [x;y;vX;vY]

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