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diff --git a/slideshow/source/inc/expressionnode.hxx b/slideshow/source/inc/expressionnode.hxx
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+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/*
+ * This file is part of the LibreOffice project.
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * This file incorporates work covered by the following license notice:
+ *
+ *   Licensed to the Apache Software Foundation (ASF) under one or more
+ *   contributor license agreements. See the NOTICE file distributed
+ *   with this work for additional information regarding copyright
+ *   ownership. The ASF licenses this file to you under the Apache
+ *   License, Version 2.0 (the "License"); you may not use this file
+ *   except in compliance with the License. You may obtain a copy of
+ *   the License at http://www.apache.org/licenses/LICENSE-2.0 .
+ */
+
+#ifndef INCLUDED_SLIDESHOW_SOURCE_INC_EXPRESSIONNODE_HXX
+#define INCLUDED_SLIDESHOW_SOURCE_INC_EXPRESSIONNODE_HXX
+
+/* Definition of ExpressionNode interface */
+
+namespace slideshow::internal
+    {
+        /** Interface describing an abstract animation function.
+
+            Use this interface to model time-dependent animation
+            functions of one variable.
+         */
+        class AnimationFunction
+        {
+        public:
+            virtual ~AnimationFunction() {}
+
+            /** Operator to calculate function value.
+
+                This method calculates the function value for the
+                given time instant t.
+
+                @param t
+                Current time instant, must be in the range [0,1]
+
+                @return the function value, typically in relative
+                user coordinate space ([0,1] range).
+             */
+            virtual double operator()( double t ) const = 0;
+
+        };
+
+        /** Refinement of AnimationFunction
+
+            This interface is used by the SMIL function parser, to
+            collapse constant values into precalculated, single nodes.
+        */
+        class ExpressionNode : public AnimationFunction
+        {
+        public:
+            /** Predicate whether this node is constant.
+
+                This predicate returns true, if this node is
+                neither time- nor ViewInfo dependent. This allows
+                for certain optimizations, i.e. not the full
+                expression tree needs be represented by
+                ExpressionNodes.
+
+                @returns true, if this node is neither time- nor
+                ViewInfo dependent
+            */
+            virtual bool isConstant() const = 0;
+        };
+
+}
+
+#endif // INCLUDED_SLIDESHOW_SOURCE_INC_EXPRESSIONNODE_HXX
+
+/* vim:set shiftwidth=4 softtabstop=4 expandtab: */