## Copyright (C) 1996 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{retval} =} is_siso (@var{sys}) 
## return nonzero if the system data structure 
## @var{sys} is single-input, single-output.
## @end deftypefn
 
## a s hodel July 1996, 1998

function  SISO = is_siso (sys)

  if(nargin != 1)
    usage("SISO = is_siso(sys)");
  elseif( !is_struct(sys))
    error("input must be a system structure (see ss2sys, tf2sys, zp2sys)");
  endif

  [n,nz,m,p] = sysdimensions(sys);

  SISO = (m == 1 & p == 1);

endfunction