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diff --git a/src/engine/Stateful.hpp b/src/engine/Stateful.hpp
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+/*
+  This file is part of Machina.
+  Copyright 2007-2013 David Robillard <http://drobilla.net>
+
+  Machina 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 any later version.
+
+  Machina 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 Machina.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#ifndef MACHINA_STATEFUL_HPP
+#define MACHINA_STATEFUL_HPP
+
+#include <stdint.h>
+
+#include "sord/sordmm.hpp"
+
+#include "machina/Atom.hpp"
+#include "machina/types.hpp"
+
+namespace Raul {
+class Atom;
+}
+
+namespace machina {
+
+class URIs;
+
+class Stateful
+{
+public:
+	Stateful();
+
+	virtual ~Stateful() {}
+
+	virtual void set(URIInt key, const Atom& value) {}
+	virtual void write_state(Sord::Model& model) = 0;
+
+	uint64_t          id() const { return _id; }
+	const Sord::Node& rdf_id(Sord::World& world) const;
+
+	static uint64_t next_id() { return _next_id++; }
+
+protected:
+	explicit Stateful(uint64_t id) : _id(id) {}
+
+private:
+	static uint64_t _next_id;
+
+	uint64_t           _id;
+	mutable Sord::Node _rdf_id;
+};
+
+class StatefulKey : public Stateful
+{
+public:
+	explicit StatefulKey(uint64_t id) : Stateful(id) {}
+
+	void write_state(Sord::Model& model) {}
+};
+
+} // namespace machina
+
+#endif // MACHINA_STATEFUL_HPP