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+## Copyright (C) 2008, 2009  VZLU Prague, a.s., Czech Republic
+## 
+## 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.
+## 
+## 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 software; see the file COPYING.  If not, see
+## <http://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## @deftypefn{Function File} {ppd =} ppder (pp)
+## Computes the piecewise derivative of a piecewise polynomial struct @var{pp}.
+## @seealso{mkpp,ppval}
+## @end deftypefn
+
+function ppd = ppder (pp)
+  if (nargin != 1)
+    print_usage ();
+  endif
+  if (! isstruct (pp))
+    error ("ppder: expects a pp structure");
+  endif
+
+  [x, p, n, k, d] = unmkpp (pp);
+  p = reshape (p, [], k);
+  if (k <= 1)
+    pd = zeros (rows (p), 1);
+    k = 1;
+  else
+    k -= 1;
+    pd = p(:,1:k) * diag (k:-1:1);
+  endif
+  ppd = mkpp (x, pd, d);
+endfunction
+