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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} =} sysappend (@var{sys}, @var{b}@{, @var{c}, @var{d}, @var{outname}, @var{inname}, @var{yd}@})
+## appends new inputs and/or outputs to a system
+##
+## @strong{Inputs}
+## @table @var
+## @item sys
+## system data structure
+##
+## @item b
+## matrix to be appended to sys "B" matrix (empty if none)
+##
+## @item c
+## matrix to be appended to sys "C" matrix (empty if none)
+##
+## @item d
+## revised sys d matrix (can be passed as [] if the revised d is all zeros)
+##
+## @item outname
+## list of names for new outputs
+##
+## @item inname
+## list of names for new inputs
+##
+## @item yd
+## binary vector; @math{yd(ii)=0} indicates a continuous output;
+## @math{yd(ii)=1} indicates a discrete output.
+## @end table
+##
+## @strong{Outputs} @var{sys}
+## @example
+## @group
+##    sys.b := [sys.b , b]
+##    sys.c := [sys.c  ]
+##             [ c     ]
+##    sys.d := [sys.d | D12 ]
+##             [D21   | D22 ]
+## @end group
+## @end example
+## where @var{D12}, @var{D21}, and @var{D22} are the appropriate dimensioned
+## blocks of the input parameter @var{d}.
+## @itemize @bullet
+## @item The leading block @var{D11} of @var{d} is ignored.
+## @item If @var{inname} and @var{outname} are not given as arguments,
+##      the new inputs and outputs are be assigned default names.
+## @item @var{yd} is a binary vector of length rows(c) that indicates
+##      continuous/sampled outputs.  Default value for @var{yd} is:
+##
+## @item @var{sys} = continuous or mixed
+## @var{yd} = @code{zeros(1,rows(c))}
+##
+## @item @var{sys} = discrete
+## @var{yd} = @code{ones(1,rows(c))}
+## @end itemize
+## @end deftypefn
+
+## Author: John Ingram <ingraje@eng.auburn.edu>
+## Created: August 1996
+
+function retsys = sysappend (sys, b, c, d, outname, inname, yd)
+
+  sav_empty_list_elements_ok = empty_list_elements_ok;
+
+  empty_list_elements_ok = 1;
+
+  ## check input arguments
+  if ( (nargin < 2) | (nargin > 7) | (!is_struct(sys)))
+    usage("retsys = sysappend(sys,b,c[,d,outname,inname,yd]) ");
+  elseif(!is_struct(sys))
+    error("sys must be a system data structure");
+  endif
+
+  ## default system type must be state space form
+  [Aa,Ab,Ac,Ad,Ats,Ann,Anz,Ast,Ain,Aout,Ayd] = sys2ss(sys);
+  [Ann,Anz,Am,Ap] = sysdimensions(sys);
+
+  ## default c
+  if(nargin < 3)      c = [];                                endif
+
+  ## default d
+  if(nargin < 4)     make_d = 1;
+  elseif(isempty(d)) make_d = 1;
+  else               make_d = 0;                             endif
+  if(make_d)         d = zeros(rows(c)+Ap,columns(b) + Am);  endif
+
+  ## Append new input(s) if any
+  Bm = max(columns(d),columns(b)+Am);
+  if(Bm != Am)
+    ## construct new signal names
+    if(nargin >= 6)   # new names were passed
+      if(!isstr(inname))
+        error("inname must be a string");
+      elseif(rows(inname) != (Bm - Am))
+        error(sprintf("%d new inputs requested; inname(%dx%d)", ...
+          (Bm-Am),rows(inname),columns(inname)));
+      endif
+    else
+      inname = sysdefioname(Bm,"u",(Am+1));
+    endif
+    if(Am)   Ain = append(Ain,inname);
+    else     Ain = inname;              endif
+
+    ## default b matrix
+    if(isempty(b))     b  = zeros(Ann+Anz,(Bm-Am));
+    elseif(rows(b) != Ann+Anz | columns(b) != (Bm-Am))
+        error(sprintf("b(%dx%d); should be (%dx%d)", rows(b), columns(b), ...
+          (Ann+Anz), (Bm-Am)));
+    endif
+
+    ## append new b matrix
+    Ab = [Ab,b];    # empty_list_elements_ok=1 makes this ok
+  endif
+
+  ## Append new output(s) if any
+  Bp = max(rows(d),rows(c)+Ap);
+  if(Bp != Ap)
+
+    ## construct new signal names, output classification
+    if(nargin >= 5)  # new names were passed
+      if(!isstr(outname))
+        error("outname must be a string");
+      elseif(rows(outname) != (Bp - Ap))
+        error(sprintf("%d new outputs requested; outname(%dx%d)", ...
+          (Bp-Ap),rows(outname),columns(outname)));
+      endif
+    else
+      outname = sysdefioname(Bp,"y",(Ap+1));
+    endif
+    if(Ap)   Aout = append(Aout,outname);
+    else     Aout = outname;                endif
+
+    ## construct new yd entries
+    if(nargin == 7)
+      if(!is_vector(yd))
+        error(sprintf("yd(%dx%d) must be a vector",rows(yd),columns(yd)))
+      elseif(rows(c) != length(yd) & rows(d) != length(yd))
+        error(sprintf("length(yd) = %d; c(%dx%d), d(%dx%d); mismatch", ...
+          length(yd), rows(c), columns(c),rows(d),columns(d)));
+      endif
+    else
+      ## default yd values
+      yd = ones(1,Bp)*( (Ats > 0) & (Ann == 0)  & isempty(find(Ayd == 0)) ) ;
+    endif
+    Ayd = [vec(Ayd);vec(yd)];
+
+    ## default c matrix
+    if(isempty(c))      c = zeros((Bp-Ap),Ann+Anz);
+    elseif(columns(c) != Ann+Anz | rows(c) != (Bp-Ap))
+        error(sprintf("c(%dx%d); should be (%dx%d)", rows(c), columns(c), ...
+          (Bp-Ap), (Ann+Anz) ));
+    endif
+
+    ## append new c matrix
+    Ac = [Ac;c];    # empty_list_elements_ok=1 makes this ok
+  endif
+
+  ## check d matrix
+  if(isempty(d)) d = zeros(Bp,Bm);
+  elseif(rows(d) != Bp | columns(d) != Bm)
+    error(sprintf("d(%dx%d) should be (%dx%d)",rows(d), columns(d), Bp, Bp));
+  endif
+
+  ## Splice in original D matrix
+  if(Am & Ap)          d(1:Ap, 1:Am) = Ad;       endif
+  Ad = d;
+
+  ## construct return system
+  retsys = ss2sys(Aa,Ab,Ac,Ad,Ats,Ann,Anz,Ast,Ain,Aout,find(Ayd == 1));
+
+  empty_list_elements_ok = sav_empty_list_elements_ok;
+
+endfunction