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diff --git a/src/mongo/db/update/update_node_visitor.h b/src/mongo/db/update/update_node_visitor.h
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+/**
+ *    Copyright (C) 2019-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    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
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#pragma once
+
+#include "mongo/platform/basic.h"
+
+namespace mongo {
+
+class AddToSetNode;
+class ArithmeticNode;
+class BitNode;
+class CompareNode;
+class ConflictPlaceholderNode;
+class CurrentDateNode;
+class ObjectReplaceNode;
+class PopNode;
+class PullAllNode;
+class PullNode;
+class PushNode;
+class RenameNode;
+class SetElementNode;
+class SetNode;
+class UnsetNode;
+class UpdateArrayNode;
+class UpdateObjectNode;
+
+/**
+ * This is a base class to allow for traversal of an update tree. It implements the visitor
+ * pattern, in which every derived class from update node implements an acceptVisitor() method,
+ * which simply calls the appropriate visit() method on the derived UpdateNodeVisitor class. The
+ * derived class can do whatever it needs to do for each specific node type in the corresponding
+ * visit() method.
+ * Derived classes are responsible for making the recursive calls to visit() if they wish
+ * to visit all the nodes in the update node tree. UpdateNodeVisitor's purpose is not actually to
+ * ensure that every node in the tree is visited, but rather to handle dynamic dispatch without
+ * having to add virtual methods to the UpdateNode interface itself.
+ */
+class UpdateNodeVisitor {
+public:
+    virtual ~UpdateNodeVisitor() = default;
+    virtual void visit(AddToSetNode*) = 0;
+    virtual void visit(ArithmeticNode*) = 0;
+    virtual void visit(BitNode*) = 0;
+    virtual void visit(CompareNode*) = 0;
+    virtual void visit(ConflictPlaceholderNode*) = 0;
+    virtual void visit(CurrentDateNode*) = 0;
+    virtual void visit(ObjectReplaceNode*) = 0;
+    virtual void visit(PopNode*) = 0;
+    virtual void visit(PullAllNode*) = 0;
+    virtual void visit(PullNode*) = 0;
+    virtual void visit(PushNode*) = 0;
+    virtual void visit(RenameNode*) = 0;
+    virtual void visit(SetElementNode*) = 0;
+    virtual void visit(SetNode*) = 0;
+    virtual void visit(UnsetNode*) = 0;
+    virtual void visit(UpdateArrayNode*) = 0;
+    virtual void visit(UpdateObjectNode*) = 0;
+};
+}  // namespace mongo