```--- a
+++ b/inst/@gf/diag.m
@@ -0,0 +1,53 @@
+## Copyright (C) 2011 David Bateman
+##
+## This program is free software; you can redistribute it and/or modify
+## the Free Software Foundation; either version 2 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} {} diag (@var{v}, @var{k})
+## Return a diagonal matrix with Galois vector @var{v} on diagonal @var{k}.
+## The second argument is optional.  If it is positive, the vector is placed on
+## the @var{k}-th super-diagonal.  If it is negative, it is placed on the
+## @var{-k}-th sub-diagonal.  The default value of @var{k} is 0, and the
+## vector is placed on the main diagonal.  For example,
+##
+## @example
+## diag (gf([1, 2, 3],2), 1)
+## ans =
+## GF(2^2) array. Primitive Polynomial = D^2+D+1 (decimal 7)
+##
+## Array elements =
+##
+##   0  1  0  0
+##   0  0  2  0
+##   0  0  0  3
+##   0  0  0  0
+## @end example
+## @end deftypefn
+
+function y = diag (g, varargin)
+  y = g;
+  y._x = diag (y._x, varargin{:});
+endfunction
+
+%!assert(diag(gf([1; 2; 3], 3)), gf([1, 0, 0; 0, 2, 0; 0, 0, 3], 3));
+%!assert(diag (gf([1; 2; 3], 3), 1), gf([0, 1, 0, 0; 0, 0, 2, 0; 0, 0, 0, 3; 0, 0, 0, 0], 3));
+%!assert(diag (gf([1; 2; 3], 3), 2), gf([0, 0, 1, 0, 0; 0, 0, 0, 2, 0; 0, 0, 0, 0, 3; 0, 0, 0, 0, 0; 0, 0, 0, 0, 0], 3));
+%!assert(diag (gf([1; 2; 3], 3),-1), gf([0, 0, 0, 0; 1, 0, 0, 0; 0, 2, 0, 0; 0, 0, 3, 0], 3));
+%!assert(diag (gf([1; 2; 3], 3),-2), gf([0, 0, 0, 0, 0; 0, 0, 0, 0, 0; 1, 0, 0, 0, 0; 0, 2, 0, 0, 0; 0, 0, 3, 0, 0], 3));
+%!assert(diag (gf([1, 0, 0; 0, 2, 0; 0, 0, 3], 3)), gf([1; 2; 3], 3));
+%!assert(diag (gf([0, 1, 0, 0; 0, 0, 2, 0; 0, 0, 0, 3; 0, 0, 0, 0], 3), 1), gf([1; 2; 3],3));
+%!assert(diag (gf([0, 0, 0, 0; 1, 0, 0, 0; 0, 2, 0, 0; 0, 0, 3, 0], 3), -1), gf([1; 2; 3], 3));
+%!assert(diag (gf(ones(1, 0), 3), 2), gf(zeros (2), 3));
+%!assert(diag (gf(1:3, 3), 4, 2), gf([1, 0; 0, 2; 0, 0; 0, 0], 3));
+
```