--- a
+++ b/inst/@quaternion/sum.m
@@ -0,0 +1,42 @@
+## Copyright (C) 2010-2013   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 -*-
+## @deftypefn {Function File} {@var{q} =} sum (@var{q})
+## @deftypefnx {Function File} {@var{q} =} sum (@var{q}, @var{dim})
+## @deftypefnx {Function File} {@var{q} =} sum (@dots{}, @var{'native'})
+## @deftypefnx {Function File} {@var{q} =} sum (@dots{}, @var{'double'})
+## @deftypefnx {Function File} {@var{q} =} sum (@dots{}, @var{'extra'})
+## Sum of elements along dimension @var{dim}.  If @var{dim} is omitted,
+## it defaults to the first non-singleton dimension.
+## See @code{help sum} for more information.
+## @end deftypefn
+
+## Author: Lukas Reichlin <lukas.reichlin@gmail.com>
+## Created: December 2013
+## Version: 0.1
+
+function q = sum (q, varargin)
+
+  if (! isa (q, "quaternion"))
+    print_usage ();
+  endif
+
+  q.w = sum (q.w, varargin{:});
+  q.x = sum (q.x, varargin{:});
+  q.y = sum (q.y, varargin{:});
+  q.z = sum (q.z, varargin{:});
+
+endfunction