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+## Copyright (C) 1996 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{num}, @var{den}, @var{tsam}, @var{inname}, @var{outname}] =} sys2tf (@var{sys})
+## Extract transfer function data from a system data structure
+##
+## See @ref{tf2sys} for parameter descriptions.
+##
+## @strong{Example}
+## @example
+## octave:1> sys=ss2sys([1 -2; -1.1,-2.1],[0;1],[1 1]);
+## octave:2> [num,den] = sys2tf(sys)
+## num = 1.0000  -3.0000
+## den = 1.0000   1.1000  -4.3000
+## @end example
+## @end deftypefn
+
+## Author: R. Bruce Tenison <btenison@eng.auburn.edu>
+## Created: June 24, 1994
+## modified to make sys2tf by A. S. Hodel Aug 1995
+## modified again for updated system format by John Ingram July 1996
+
+function [num, den, tsam, inname, outname] = sys2tf (Asys)
+
+  if(nargin != 1)
+    usage("[num,den,tsam,inname,outname] = sys2tf(Asys)");
+  endif
+
+  if( !is_struct(Asys))
+    error("Asys must be a system data structure (see ss2sys, tf2sys, zp2sys)");
+  elseif (! is_siso(Asys) )
+    [n, nz, m, p] = sysdimensions(Asys);
+    error(["system is not SISO (",num2str(m)," inputs, ...
+        ", num2str(p)," outputs"]);
+  endif
+
+  Asys = sysupdate(Asys,"tf");          # just in case
+
+  num = Asys.num;
+  den = Asys.den;
+
+  tsam = sysgettsam(Asys);
+  inname = sysgetsignals(Asys,"in");
+  outname = sysgetsignals(Asys,"out");
+
+endfunction
+