Capacitor:

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   Dynamical model for a capacitor.
   It determines the voltage derivative from inductance and current. 

   i = c dv/dt

   Returns dv/dt or the state space matrices [a, b, c, d]

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   Form:
   [a, b, c, d] = Capacitor( c )

   or

   iDot         = Capacitor( g, i )

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   ------
   Inputs
   ------
   g          (1,1)  Data structure
                     .c        (1,1) Capacitance
   i          (1,1)  Current

   -------
   Outputs
   -------
   iDot pr a  (1,1)  Voltage derivative or state matrix
   b          (1,1)  Input matrix
   c          (1,1)  Output matrix
   d          (1,1)  Feedthrough matrix

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