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+## Copyright (C) 1996, 1998, 1999 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{sys} =} sysgroup (@var{Asys}, @var{Bsys})
+## Combines two systems into a single system
+##
+## @strong{Inputs}
+## @var{Asys}, @var{Bsys}: system data structures
+##
+## @strong{Outputs}
+## @math{sys = @r{block diag}(Asys,Bsys)}
+## @example
+## @group
+##          __________________
+##          |    ________    |
+## u1 ----->|--> | Asys |--->|----> y1
+##          |    --------    |
+##          |    ________    |
+## u2 ----->|--> | Bsys |--->|----> y2
+##          |    --------    |
+##          ------------------
+##               Ksys
+## @end group
+## @end example
+## The function also rearranges the internal state-space realization of @var{sys}
+## so that the
+## continuous states come first and the discrete states come last.
+## If there are duplicate names, the second name has a unique suffix appended
+## on to the end of the name.
+## @end deftypefn
+
+## Author: A. S. Hodel <a.s.hodel@eng.auburn.edu>
+## Created: August 1995
+## modified by John Ingram July 1996
+## A. S. Hodel: modified for variable number of arguments 1999
+
+function sys = sysgroup (...)
+
+  save_emp = empty_list_elements_ok;
+  empty_list_elements_ok = 1;
+
+  if(nargin < 1)
+    usage("sys = sysgroup(Asys{,Bsys,...})");
+  endif
+
+  ## collect all arguments
+  arglist = list();
+  va_start();
+  for kk=1:nargin
+    arglist(kk) = va_arg();
+    if(!is_struct(nth(arglist,kk)))
+      error("sysgroup: argument %d is not a data structure",kk);
+    endif
+  endfor
+
+  if(nargin == 2)
+    ## the usual case; group the two systems together
+    Asys = nth(arglist,1);
+    Bsys = nth(arglist,2);
+
+    ## extract information from Asys, Bsys to consruct sys
+    Asys = sysupdate(Asys,"ss");
+    Bsys = sysupdate(Bsys,"ss");
+    [n1,nz1,m1,p1] = sysdimensions(Asys);
+    [n2,nz2,m2,p2] = sysdimensions(Bsys);
+    [Aa,Ab,Ac,Ad,Atsam,An,Anz,Ast,Ain,Aout,Ayd] = sys2ss(Asys);
+    [Ba,Bb,Bc,Bd,Btsam,Bn,Bnz,Bst,Bin,Bout,Byd] = sys2ss(Bsys);
+    nA = An + Anz;
+    nB = Bn + Bnz;
+
+    if(p1*m1*p2*m2 == 0)
+      error("sysgroup: argument lacks inputs and/or outputs");
+
+    elseif((Atsam + Btsam > 0) & (Atsam * Btsam == 0) )
+      warning("sysgroup: creating combination of continuous and discrete systems")
+
+    elseif(Atsam != Btsam)
+      error("sysgroup: Asys.tsam=%e, Bsys.tsam =%e", Atsam, Btsam);
+    endif
+
+    A = [Aa,zeros(nA,nB); zeros(nB,nA),Ba];
+    B = [Ab,zeros(nA,m2); zeros(nB,m1),Bb];
+    C = [Ac,zeros(p1,nB); zeros(p2,nA),Bc];
+    D = [Ad,zeros(p1,m2); zeros(p2,m1),Bd];
+    tsam = max(Atsam,Btsam);
+
+    ## construct combined signal names; stnames must check for pure gain blocks
+    if(isempty(Ast))
+      stname = Bst;
+    elseif(isempty(Bst))
+      stname = Ast;
+    else
+      stname  = append(Ast, Bst);
+    endif
+    inname  = append(Ain, Bin);
+    outname = append(Aout,Bout);
+
+    ## Sort states into continous first, then discrete
+    dstates = ones(1,(nA+nB));
+    if(An)
+      dstates(1:(An)) = zeros(1,An);
+    endif
+    if(Bn)
+      dstates((nA+1):(nA+Bn)) = zeros(1,Bn);
+    endif
+    [tmp,pv] = sort(dstates);
+    A = A(pv,pv);
+    B = B(pv,:);
+    C = C(:,pv);
+    stname = stname(pv);
+
+    ## check for duplicate signal names
+    inname = sysgroupn(inname,"input");
+    stname = sysgroupn(stname,"state");
+    outname = sysgroupn(outname,"output");
+
+    ## mark discrete outputs
+    outlist = find([Ayd, Byd]);
+
+    ## build new system
+    sys = ss2sys(A,B,C,D,tsam,An+Bn,Anz+Bnz,stname,inname,outname);
+
+  else
+    ## multiple systems (or a single system); combine together one by one
+    sys = nth(arglist,1);
+    for kk=2:length(arglist)
+      printf("sysgroup: kk=%d\n",kk);
+      sys = sysgroup(sys,nth(arglist,kk));
+    endfor
+  endif
+
+  empty_list_elements_ok = save_emp;
+
+endfunction