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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} =} sysprune (@var{Asys}, @var{out_idx}, @var{in_idx})
+## Extract specified inputs/outputs from a system
+##
+## @strong{Inputs}
+## @table @var
+## @item   Asys
+## system data structure
+## @item out_idx
+## @itemx in_idx
+## list of connections indices; the new
+## system has outputs y(out_idx(ii)) and inputs u(in_idx(ii)).
+## May select as [] (empty matrix) to specify all outputs/inputs.
+## @end table
+##
+## @strong{Outputs}
+## @var{retsys}: resulting system
+## @example
+## @group
+##            ____________________
+## u1 ------->|                  |----> y1
+##  (in_idx)  |       Asys       | (out_idx)
+## u2 ------->|                  |----| y2
+##   (deleted)-------------------- (deleted)
+## @end group
+## @end example
+## @end deftypefn
+
+## Author: A. S. Hodel <a.s.hodel@eng.auburn.edu>
+## Created: August 1995
+## Updated by John Ingram 7-15-96
+
+function sys = sysprune (sys, output_idx, input_idx, state_idx)
+
+  if( nargin < 3 | nargin > 4  )
+    usage("retsys = sysprune(sys,output_idx,input_idx{,state_idx})");
+  elseif(nargin < 4)
+    state_idx = [];
+  endif
+
+  ## default: no action
+  [nn,nz,mm,pp] = sysdimensions(sys);
+  if(isempty(output_idx)) output_idx = 1:pp; endif
+  if(isempty(input_idx)) input_idx = 1:mm; endif
+  if(isempty(state_idx)) state_idx = 1:(nn+nz); endif
+
+  ## check dimensions
+  if( !(is_vector(output_idx) | isempty(output_idx) )  )
+    if(!is_matrix(output_idx))
+      error("sysprune: bad argument passed for output_idx");
+    else
+      error("sysprune: output_idx (%d x %d) must be a vector or empty", ...
+        rows(output_idx),columns(output_idx));
+    endif
+  elseif(is_duplicate_entry(output_idx))
+     error("sysprune: duplicate entries found in output_idx");
+  endif
+
+  if( !(is_vector(input_idx) | isempty(input_idx) )  )
+    if(!is_matrix(input_idx))
+      error("sysprune: bad argument passed for input_idx");
+    else
+      error("sysprune: input_idx (%d x %d) must be a vector or empty", ...
+        rows(input_idx),columns(input_idx));
+    endif
+  elseif(is_duplicate_entry(input_idx))
+     error("sysprune: duplicate entries found in input_idx");
+  endif
+
+  if( !(is_vector(state_idx) | isempty(state_idx) )  )
+    if(!is_matrix(state_idx))
+      error("sysprune: bad argument passed for state_idx");
+    else
+      error("sysprune: state_idx (%d x %d) must be a vector or empty", ...
+        rows(state_idx),columns(state_idx));
+    endif
+  elseif(nn+nz > 0)
+    if(is_duplicate_entry(state_idx))
+      error("sysprune: duplicate entries found in state_idx");
+    endif
+  endif
+
+  lo = length(output_idx);
+  li = length(input_idx);
+  lst = length(state_idx);
+
+  if( !is_struct(sys))
+    error("Asys must be a system data structure (see ss2sys, tf2sys, or zp2sys)")
+  elseif(pp < lo)
+    error([num2str(lo)," output_idx entries, system has only ", ...
+        num2str(pp)," outputs"]);
+  elseif(mm < li)
+    error([num2str(li)," input_idx entries, system has only ", ...
+        num2str(mm)," inputs"]);
+  elseif(nn+nz < lst)
+    error([num2str(lst)," state_idx entries, system has only ", ...
+        num2str(nn+nz)," states"]);
+  endif
+
+  [aa,bb,cc,dd,tsam,nn,nz,stnam,innam,outnam,yd] = sys2ss(sys);
+
+  ## check for valid state permutation
+  if(nn & nz)
+    c_idx = find(state_idx <= nn);
+    if(!isempty(c_idx)) max_c = max(c_idx);
+    else                max_c = 0;            endif
+    d_idx = find(state_idx > nn);
+    if(!isempty(d_idx)) min_d = min(d_idx);
+    else                min_d = nn+nz;            endif
+    if(max_c > min_d)
+      warning("sysprune: state_idx(%d)=%d (discrete) preceeds", ...
+        min_d,state_idx(min_d));
+      warning("          state_idx(%d)=%d (continuous)",...
+        max_c,state_idx(max_c));
+      warning("sysprune: sys has %d continuous states, %d discrete states", ...
+        nn,nz);
+      error("continuous/discrete state partition not preserved ; see ss2sys");
+    endif
+  endif
+
+  idx = input_idx;
+  odx = output_idx;
+  if(isempty(state_idx))
+    idx = [];
+    odx = [];
+  endif
+  aa = aa(state_idx,state_idx);
+  bb = bb(state_idx,idx);
+  cc = cc(odx,state_idx);
+  dd = dd(output_idx,input_idx);
+  yd = yd(output_idx);
+
+  innam  = innam(input_idx);
+  outnam = outnam(output_idx);
+  stnam = stnam(state_idx);
+  nn1 = length(find(state_idx <= nn));
+  nz1 = length(find(state_idx > nn));
+  sys = ss2sys(aa,bb,cc,dd,tsam,nn1,nz1,stnam,innam,outnam,find(yd));
+endfunction