SSEmpty:

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   Creates an empty state space system.
   a = [], b = [], c = [], d = 1;
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   Form:
   [a, b, c, d] = SSEmpty
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   ------
   Inputs
   ------
   None

   -------
   Outputs
   -------
   a                      Plant matrix
   b                      Input matrix
   c                      Output matrix
   d                      Feedthrough matrix

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