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+## Copyright (C) 1996, 1998 Auburn University.  All rights reserved.
+##
+## This file is part of Octave.
+##
+## Octave is free software; you can redistribute it and/or modify it
+## under the terms of the GNU General Public License as published by the
+## Free Software Foundation; either version 2, or (at your option) any
+## later version.
+##
+## Octave is distributed in the hope that it will be useful, but WITHOUT
+## ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+## FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+## for more details.
+##
+## You should have received a copy of the GNU General Public License
+## along with Octave; see the file COPYING.  If not, write to the Free
+## Software Foundation, 59 Temple Place, Suite 330, Boston, MA 02111 USA.
+
+## -*- texinfo -*-
+## @deftypefn {Function File} {[@var{a}, @var{b}, @var{c}, @var{d}, @var{tsam},@var{n}, @var{nz}, @var{stname}, @var{inname}, @var{outname}, @var{yd}] =} sys2ss (@var{sys})
+## Extract state space representation from system data structure.
+##
+## @strong{Inputs}
+## @var{sys} system data structure (@pxref{sysstruct})
+##
+## @strong{Outputs}
+## @table @var
+## @item a
+## @itemx b
+## @itemx c
+## @itemx d
+## state space matrices for sys
+##
+## @item tsam
+## sampling time of sys (0 if continuous)
+##
+## @item n
+## @itemx nz
+## number of continuous, discrete states (discrete states come
+## last in state vector @var{x})
+##
+## @item stname
+## @itemx inname
+## @itemx outname
+## signal names (lists of strings);  names of states,
+## inputs, and outputs, respectively
+##
+## @item yd
+## binary vector; @var{yd}(@var{ii}) is 1 if output @var{y}(@var{ii})$
+## is discrete (sampled); otherwise  @var{yd}(@var{ii}) 0.
+##
+## @end table
+## A warning massage is printed if the system is a mixed
+## continuous and discrete system
+##
+## @strong{Example}
+## @example
+## octave:1> sys=tf2sys([1 2],[3 4 5]);
+## octave:2> [a,b,c,d] = sys2ss(sys)
+## a =
+##    0.00000   1.00000
+##   -1.66667  -1.33333
+## b =
+##   0
+##   1
+## c = 0.66667  0.33333
+## d = 0
+## @end example
+## @end deftypefn
+
+## Author: David Clem
+## Created: August 19, 1994
+## Updates by John Ingram July 14, 1996
+
+function [a, b, c, d, tsam, n, nz, stname, inname, outname, yd] = sys2ss (sys)
+
+  if(nargin != 1)
+    usage("[a,b,c,d,tsam,n,nz,stname,inname,outname,yd] = sys2ss(sys)")
+  endif
+
+  if (nargout > 11)
+    warning(["sys2ss: ",num2str(nargout)," out arguments exceeds max=11"])
+    usage("[a,b,c,d,tsam,n,nz,stname,inname,outname,yd] = sys2ss(sys)")
+  endif
+
+  if( ! is_struct(sys) )
+    error("input argument must be a system data structure");
+  endif
+
+  sys = sysupdate(sys,"ss");        # make sure state space data is there
+  [n,nz,m,p] = sysdimensions(sys);
+  [stname,inname,outname,yd] = sysgetsignals(sys);
+  tsam = sysgettsam(sys);
+
+  cont = sum(yd == 0) + n;
+  dig = sum(yd != 0) + nz + tsam;
+  if(cont*dig)
+    warning("sys2ss: input system is mixed continuous/discrete");
+  endif
+
+  a = sys.a;
+  b = sys.b;
+  c = sys.c;
+  d = sys.d;
+
+endfunction
+