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diff --git a/octave_packages/control-2.3.52/@lti/d2c.m b/octave_packages/control-2.3.52/@lti/d2c.m
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+## Copyright (C) 2011   Lukas F. Reichlin
+##
+## This file is part of LTI Syncope.
+##
+## LTI Syncope 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.
+##
+## LTI Syncope 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 LTI Syncope.  If not, see <http://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## @deftypefn {Function File} {@var{sys} =} d2c (@var{sys})
+## @deftypefnx {Function File} {@var{sys} =} d2c (@var{sys}, @var{method})
+## @deftypefnx {Function File} {@var{sys} =} d2c (@var{sys}, @var{"prewarp"}, @var{w0})
+## Convert the discrete lti model into its continuous-time equivalent.
+##
+## @strong{Inputs}
+## @table @var
+## @item sys
+## Discrete-time LTI model.
+## @item method
+## Optional conversion method.  If not specified, default method @var{"zoh"}
+## is taken.
+## @table @var
+## @item "zoh"
+## Zero-order hold or matrix logarithm.
+## @item "tustin", "bilin"
+## Bilinear transformation or Tustin approximation.
+## @item "prewarp"
+## Bilinear transformation with pre-warping at frequency @var{w0}.
+## @end table
+## @end table
+##
+## @strong{Outputs}
+## @table @var
+## @item sys
+## Continuous-time LTI model.
+## @end table
+## @end deftypefn
+
+## Author: Lukas Reichlin <lukas.reichlin@gmail.com>
+## Created: September 2011
+## Version: 0.2
+
+function sys = d2c (sys, method = "std", w0 = 0)
+
+  if (nargin == 0 || nargin > 3)
+    print_usage ();
+  endif
+
+  if (! isa (sys, "lti"))
+    error ("d2c: first argument is not an lti model");
+  endif
+
+  if (isct (sys))
+    error ("d2c: system is already continuous-time");
+  endif
+
+  if (! ischar (method))
+    error ("d2c: second argument is not a string");
+  endif
+
+  if (! issample (w0, 0))
+    error ("d2c: third argument is not a valid pre-warping frequency");
+  endif
+
+  sys = __d2c__ (sys, sys.tsam, lower (method), w0);
+  sys.tsam = 0;
+
+endfunction
+
+
+## bilinear transformation
+## both directions
+%!shared Mo, Me
+%! A = [  1.0  0.5
+%!        0.5  1.0 ];
+%!
+%! B = [  0.0 -1.0
+%!        1.0  0.0 ];
+%!
+%! C = [ -1.0  0.0
+%!        0.0  1.0 ];
+%!
+%! D = [  1.0  0.0
+%!        0.0 -1.0 ];
+%!
+%! [Ao, Bo, Co, Do] = ssdata (d2c (c2d (ss (A, B, C, D), 2, "tustin"), "tustin"));
+%!
+%! Mo = [Ao, Bo; Co, Do];
+%! Me = [A, B; C, D];
+%!
+%!assert (Mo, Me, 1e-4);
+
+
+## zero-order hold
+## both directions
+%!shared Mo, Me
+%! A = [  1.0  0.5
+%!        0.5  1.0 ];
+%!
+%! B = [  0.0 -1.0
+%!        1.0  0.0 ];
+%!
+%! C = [ -1.0  0.0
+%!        0.0  1.0 ];
+%!
+%! D = [  1.0  0.0
+%!        0.0 -1.0 ];
+%!
+%! [Ao, Bo, Co, Do] = ssdata (d2c (c2d (ss (A, B, C, D), 2, "zoh"), "zoh"));
+%!
+%! Mo = [Ao, Bo; Co, Do];
+%! Me = [A, B; C, D];
+%!
+%!assert (Mo, Me, 1e-4);
+
+
+## bilinear transformation with pre-warping
+## both directions
+%!shared Mo, Me
+%! A = [  1.0  0.5
+%!        0.5  1.0 ];
+%!
+%! B = [  0.0 -1.0
+%!        1.0  0.0 ];
+%!
+%! C = [ -1.0  0.0
+%!        0.0  1.0 ];
+%!
+%! D = [  1.0  0.0
+%!        0.0 -1.0 ];
+%!
+%! [Ao, Bo, Co, Do] = ssdata (d2c (c2d (ss (A, B, C, D), 2, "prewarp", 1000), "prewarp", 1000));
+%!
+%! Mo = [Ao, Bo; Co, Do];
+%! Me = [A, B; C, D];
+%!
+%!assert (Mo, Me, 1e-4);