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+## Copyright (C) 2008 David Bateman
+##
+## 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 3 of the License, 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, see
+## <http://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## @deftypefn {Function File} {@var{idx} =} subsindex (@var{a})
+## Convert an object to an index vector. When @var{a} is a class object 
+## defined with a class constructor, then @code{subsindex} is the
+## overloading method that allows the conversion of this class object to
+## a valid indexing vector. It is important to note that
+## @code{subsindex} must return a zero-based real integer vector of the
+## class "double". For example, if the class constructor
+##
+## @example
+## @group
+## function b = myclass (a)
+##  b = myclass (struct ("a", a), "myclass");
+## endfunction
+## @end group
+## @end example
+##
+## @noindent
+## then the @code{subsindex} function
+##
+## @example
+## @group
+## function idx = subsindex (a)
+##  idx = double (a.a) - 1.0;
+## endfunction
+## @end group
+## @end example
+##
+## @noindent
+## can then be used as follows
+##
+## @example
+## @group
+## a = myclass (1:4);
+## b = 1:10;
+## b(a)
+## @result{} 1  2  3  4
+## @end group
+## @end example
+##
+## @seealso{class, subsref, subsasgn}
+## @end deftypefn
+
+function idx = subsindex (a)
+  error ("subsindex: not defined for class \"%s\"", class(a));
+endfunction