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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{retsys} =} sysdup (@var{Asys}, @var{out_idx}, @var{in_idx})
+## Duplicate specified input/output connections of a system
+##
+## @strong{Inputs}
+## @table @var
+## @item Asys
+## system data structure (@pxref{ss2sys})
+## @item out_idx
+## @itemx in_idx
+## list of connections indices;
+## duplicates are made of @code{y(out_idx(ii))} and @code{u(in_idx(ii))}.
+## @end table
+##
+## @strong{Outputs}
+## @var{retsys}: resulting closed loop system:
+## duplicated i/o names are appended with a @code{"+"} suffix.
+##
+##
+## @strong{Method}
+## @code{sysdup} creates copies of selected inputs and outputs as
+## shown below.  u1/y1 is the set of original inputs/outputs, and
+## u2,y2 is the set of duplicated inputs/outputs in the order specified
+## in @var{in_idx}, @var{out_idx}, respectively
+## @example
+## @group
+##           ____________________
+## u1  ----->|                  |----> y1
+##           |       Asys       |
+## u2 ------>|                  |----->y2
+## (in_idx)  -------------------| (out_idx)
+## @end group
+## @end example
+## @end deftypefn
+
+## Author: A. S. Hodel <a.s.hodel@eng.auburn.edu>
+## Created: August 1995
+## modified by John Ingram July 1996
+
+function retsys = sysdup (Asys, output_list, input_list)
+
+  if( nargin != 3)
+    usage("retsys = sysdup(Asys,output_list,input_list)");
+  endif
+
+  if( !is_struct(Asys))
+    error("Asys must be a system data structure (see ss2sys, tf2sys, or zp2sys)")
+  endif
+
+  Asys = sysupdate(Asys,"ss");
+  [nn,nz,mm,pp] = sysdimensions(Asys);
+  [aa,bb,cc,dd] = sys2ss(Asys);
+
+  ## first duplicate inputs
+  if(is_vector(input_list))
+    for ii=1:length(input_list);
+      bb(:,mm+ii) = bb(:,input_list(ii));
+      dd(:,mm+ii) = dd(:,input_list(ii));
+    end
+  elseif(!isempty(input_list))
+    error("input_list must be a vector or empty");
+  endif
+
+
+  ## now duplicate outputs
+  osize = min(size(output_list));
+  if(osize == 1)
+    for ii=1:length(output_list);
+      cc(pp+ii,:) = cc(output_list(ii),:);
+      dd(pp+ii,:) = dd(output_list(ii),:);
+    end
+  elseif(osize != 0)
+    error("output_list must be a vector or empty");
+  endif
+
+  [stnam,innam,outnam,yd] = sysgetsignals(Asys);
+  tsam = sysgettsam(Asys);
+
+  ## pack system and then rename signals
+  retsys = ss2sys(aa,bb,cc,dd,tsam,nn,nz);
+  retsys = syssetsignals(retsys,"in",innam,1:mm);
+  retsys = syssetsignals(retsys,"out",outnam,1:pp);
+  retsys = syssetsignals(retsys,"yd",yd,1:pp);
+
+  ## update added input names
+  for ii=(mm+1):(mm+length(input_list))
+    onum = input_list(ii-mm);
+    strval = sprintf("%s(dup)",sysgetsignals(retsys,"in",onum,1) );
+    retsys = syssetsignals(retsys,"in",strval,ii);
+  endfor
+
+  ## update added output names/discrete flags
+  ## give default names to the added outputs
+  for jj=(pp+1):(pp+length(output_list))
+    onum = output_list(jj-pp);
+    strval = sprintf("%s(dup)",sysgetsignals(retsys,"out",onum,1) );
+    retsys = syssetsignals(retsys,"out",strval,jj);
+    dflg = sysgetsignals(retsys,"yd",onum);
+    retsys = syssetsignals(retsys,"yd",dflg,jj);
+  endfor
+
+endfunction