Series:

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   Series connects two state space models.

   Series connects two state space models so that y2 is the output
   and u1 is the input.

   u1 ----> P1 -----> P2 -----> y2

   Since version 1.
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   Form: [a, b, c, d] = Series( a1, b1, c1, d1, a2, b2, c2, d2 )
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   ------
   Inputs
   ------
   a1                  Plant matrix
   b1                  Input matrix
   c1                  Measurement matrix
   d1                  Input feedthrough matrix
   a2                  Plant matrix
   b2                  Input matrix
   c2                  Measurement matrix
   d2                  Input feedthrough matrix

   -------
   Outputs
   -------
   a                   Plant matrix
   b                   Input matrix
   c                   Measurement matrix
   d                   Input feedthrough matrix

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	References: Maciejowski, J. M., Multivariable Feedback Design, 
               Addison-Wesley, Reading, MA, 1989, pp. 372-373.
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