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+ /*    This file is part of the Gudhi Library. The Gudhi library 
+  *    (Geometric Understanding in Higher Dimensions) is a generic C++ 
+  *    library for computational topology.
+  *
+  *    Author(s):       Clément Maria
+  *
+  *    Copyright (C) 2014  INRIA Sophia Antipolis-Méditerranée (France)
+  *
+  *    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/>.
+  */
+
+/** \brief Value type for a filtration function on a cell complex.
+  *
+  * A <EM>filtration</EM> of a cell complex (see FilteredComplex) is
+  * a function \f$f:\mathbf{K} \rightarrow \mathbb{R}\f$ satisfying \f$f(\tau)\leq 
+  * f(\sigma)\f$ whenever \f$\tau \subseteq \sigma\f$. Ordering the simplices 
+  * by increasing filtration values (breaking ties so as a simplex appears after 
+  * its subsimplices of same filtration value) provides an indexing scheme 
+  * (see IndexingTag).
+  */
+  struct FiltrationValue {
+    /** \brief Operator < is a StrictWeakOrdering. */
+    bool operator<(FiltrationValue f1, FiltrationValue f2);
+  };