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diff --git a/libyul/optimiser/Substitution.h b/libyul/optimiser/Substitution.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 copier that replaces certain identifiers with expressions.
+ */
+
+#pragma once
+
+#include <libyul/optimiser/ASTCopier.h>
+
+#include <string>
+#include <map>
+#include <set>
+
+namespace dev
+{
+namespace yul
+{
+
+/**
+ * Specific AST copier that replaces certain identifiers with expressions.
+ */
+class Substitution: public ASTCopier
+{
+public:
+	Substitution(std::map<std::string, Expression const*> const& _substitutions):
+		m_substitutions(_substitutions)
+	{}
+	virtual Expression translate(Expression const& _expression) override;
+
+private:
+	std::map<std::string, Expression const*> const& m_substitutions;
+};
+
+}
+}