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diff --git a/libjulia/optimiser/Semantics.h b/libjulia/optimiser/Semantics.h
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+/*
+	This file is part of solidity.
+
+	solidity 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.
+
+	solidity 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 solidity.  If not, see <http://www.gnu.org/licenses/>.
+*/
+/**
+ * Specific AST walkers that collect semantical facts.
+ */
+
+#pragma once
+
+#include <libjulia/optimiser/ASTWalker.h>
+
+#include <string>
+#include <map>
+#include <set>
+
+namespace dev
+{
+namespace julia
+{
+
+/**
+ * Specific AST walker that determines whether an expression is movable.
+ */
+class MovableChecker: public ASTWalker
+{
+public:
+	MovableChecker() = default;
+	explicit MovableChecker(Expression const& _expression);
+
+	virtual void operator()(Identifier const& _identifier) override;
+	virtual void operator()(FunctionalInstruction const& _functionalInstruction) override;
+	virtual void operator()(FunctionCall const& _functionCall) override;
+
+	/// Disallow visiting anything apart from Expressions (this throws).
+	virtual void visit(Statement const&) override;
+	using ASTWalker::visit;
+
+	bool movable() const { return m_movable; }
+	std::set<std::string> const& referencedVariables() const { return m_variableReferences; }
+
+private:
+	/// Which variables the current expression references.
+	std::set<std::string> m_variableReferences;
+	/// Is the current expression movable or not.
+	bool m_movable = true;
+};
+
+}
+}