--- /dev/null
+## Copyright (C) 2010 Lukas F. Reichlin
+##
+## This program 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.
+##
+## This program 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 this program. If not, see <http://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## Addition of two quaternions. Used by Octave for "q1 + q2".
+
+## Author: Lukas Reichlin <lukas.reichlin@gmail.com>
+## Created: May 2010
+## Version: 0.1
+
+function a = plus (a, b)
+
+ if (! isa (a, "quaternion"))
+ a = quaternion (a);
+ endif
+
+ if (! isa (b, "quaternion"))
+ b = quaternion (b);
+ endif
+
+ a.w = a.w + b.w;
+ a.x = a.x + b.x;
+ a.y = a.y + b.y;
+ a.z = a.z + b.z;
+
+endfunction