--- /dev/null
+## Copyright (C) 2009, 2012 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 -*-
+## Block diagonal concatenation of two TF models.
+## This file is part of the Model Abstraction Layer.
+## For internal use only.
+
+## Author: Lukas Reichlin <lukas.reichlin@gmail.com>
+## Created: September 2009
+## Version: 0.2
+
+function retsys = __sys_group__ (sys1, sys2)
+
+ if (! isa (sys1, "tf"))
+ sys1 = tf (sys1);
+ endif
+
+ if (! isa (sys2, "tf"))
+ sys2 = tf (sys2);
+ endif
+
+ retsys = tf ();
+
+ retsys.lti = __lti_group__ (sys1.lti, sys2.lti);
+
+ [p1, m1] = size (sys1);
+ [p2, m2] = size (sys2);
+
+ empty12 = tfpolyzeros (p1, m2);
+ empty21 = tfpolyzeros (p2, m1);
+
+ retsys.num = [sys1.num, empty12 ;
+ empty21, sys2.num];
+
+ empty12 = tfpolyones (p1, m2);
+ empty21 = tfpolyones (p2, m1);
+
+ retsys.den = [sys1.den, empty12 ;
+ empty21, sys2.den];
+
+ if (sys1.tfvar == sys2.tfvar)
+ retsys.tfvar = sys1.tfvar;
+ elseif (sys1.tfvar == "x")
+ retsys.tfvar = sys2.tfvar;
+ else
+ retsys.tfvar = sys1.tfvar;
+ endif
+
+ if (sys1.inv || sys2.inv)
+ retsys.inv = true;
+ endif
+
+endfunction