new file mode 100644
--- /dev/null
+++ b/scripts/control/system/sysadd.m
@@ -0,0 +1,118 @@
+## Copyright (C) 1996, 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} =}  sysadd (@var{Gsys}, @var{Hsys})
+## returns @var{sys} = @var{Gsys} + @var{Hsys}.
+## @itemize @bullet
+## @item Exits with
+## an error if @var{Gsys} and @var{Hsys} are not compatibly dimensioned.
+## @item Prints a warning message is system states have identical names;
+## duplicate names are given a suffix to make them unique.
+## @item @var{sys} input/output names are taken from @var{Gsys}.
+## @end itemize
+## @example
+## @group
+##           ________
+##      ----|  Gsys  |---
+## u   |    ----------  +|
+## -----                (_)----> y
+##     |     ________   +|
+##      ----|  Hsys  |---
+##           --------
+## @end group
+## @end example
+## @end deftypefn
+
+## Author: John Ingram <ingraje@eng.auburn.edu>
+## Created: July 1996
+## Updated for variable number of arguments July 1999 A. S. Hodel
+
+function sys = sysadd (...)
+
+  if(nargin < 1)
+    usage("sysadd: sys = sysysadd(Gsys{,Hsys, ...})");
+  endif
+
+  ## collect all arguments
+  arglist = list();
+  va_start();
+  for kk=1:nargin
+    arglist(kk) = va_arg();
+    if(!is_struct(nth(arglist,kk)))
+      error("sysadd: argument %d is not a data structure",kk);
+    endif
+  endfor
+
+  ## check system dimensions
+  [n,nz,mg,pg,Gyd] = sysdimensions(nth(arglist,1));
+  for kk=2:nargin
+    [n,nz,mh,ph,Hyd] = sysdimensions(nth(arglist,kk));
+    if(mg != mh)
+      error("arg 1 has %d inputs; arg %d has vs %d inputs",mg,kk,mh);
+    elseif(pg != ph)
+      error("arg 1 has %d outputs; arg %d has vs %d outputs",pg,kk,ph);
+    elseif(norm(Gyd - Hyd))
+      warning("cannot add a discrete output to a continuous output");
+      error("Output type mismatch: arguments 1 and %d\n",kk);
+    endif
+  endfor
+
+  ## perform the add
+  if(nargin == 2)
+    Gsys = nth(arglist,1);   Hsys = nth(arglist,2);
+    if( strcmp(sysgettype(Gsys),"tf") | strcmp(sysgettype(Hsys),"tf") )
+      ## see if adding  transfer functions with identical denominators
+      [Gnum,Gden,GT,Gin,Gout] = sys2tf(Gsys);
+      [Hnum,Hden,HT,Hin,Hout] = sys2tf(Hsys);
+      if(length(Hden) == length(Gden) )
+        if( (Hden == Gden) & (HT == GT) )
+          sys = tf2sys(Gnum+Hnum,Gden,GT,Gin,Gout);
+          return
+        endif
+        ## if not, we go on and do the usual thing...
+      endif
+    endif
+
+    ## make sure in ss form
+    Gsys = sysupdate(Gsys,"ss");
+    Hsys = sysupdate(Hsys,"ss");
+
+    ## change signal names to avoid warning messages from sysgroup
+    Gsys = syssetsignals(Gsys,"in",sysdefioname(length(Gin),"Gin_u"));
+    Gsys = syssetsignals(Gsys,"out",sysdefioname(length(Gout),"Gout_u"));
+    Hsys = syssetsignals(Hsys,"in",sysdefioname(length(Hin),"Hin_u"));
+    Hsys = syssetsignals(Hsys,"out",sysdefioname(length(Hout),"Hout_u"));
+
+    sys = sysgroup(Gsys,Hsys);
+
+    eyin = eye(mg);
+    eyout = eye(pg);
+
+    sys = sysscale(sys,[eyout, eyout],[eyin;eyin],Gout,Gin);
+
+  else
+    ## multiple systems (or a single system); combine together one by one
+    sys = nth(arglist,1);
+    for kk=2:length(arglist)
+      sys = sysadd(sys,nth(arglist,kk));
+    endfor
+  endif
+
+endfunction
+