Copyright (C) 2000, HELM Team. This file is part of HELM, an Hypertextual, Electronic Library of Mathematics, developed at the Computer Science Department, University of Bologna, Italy. HELM 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 of the License, or (at your option) any later version. HELM 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 HELM; if not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. For details, see the HELM World-Wide-Web page, http://cs.unibo.it/helm/.