DoubleIntegratorRHS:

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   Right hand side for a double integrator.

   Since version 10.
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   Form:
   xDot = DoubleIntegratorRHS( x, t, u, mass )
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   ------
   Inputs
   ------
   x	      (2,1)  [position;velocity]
   t         (1,1)  Time
   u         (1,1)  Control
   mass      (1,1)  Mass

   -------
   Outputs
   -------
   xDot      (2,1)  d[position;velocity]/dt

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