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diff --git a/src/mongo/db/ops/field_checker.h b/src/mongo/db/ops/field_checker.h
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+++ b/src/mongo/db/ops/field_checker.h
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+/**
+ *    Copyright (C) 2013 10gen Inc.
+ *
+ *    This program is free software: you can redistribute it and/or  modify
+ *    it under the terms of the GNU Affero General Public License, version 3,
+ *    as published by the Free Software Foundation.
+ *
+ *    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
+ *    GNU Affero General Public License for more details.
+ *
+ *    You should have received a copy of the GNU Affero General Public License
+ *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include "mongo/base/status.h"
+
+namespace mongo {
+
+    class FieldRef;
+
+    namespace fieldchecker {
+
+        /**
+         * Returns OK if all the below conditions on 'field' are valid:
+         *   + Non-empty
+         *   + Not the _id field (or a subfield of the _id field, such as _id.x.y)
+         *   + Does not start or end with a '.'
+         *   + Does not start with a $
+         * Otherwise returns a code indicating cause of failure.
+         */
+        Status isUpdatable(const FieldRef& field);
+
+        /**
+         * Same behavior of isUpdatable but allowing update fields to start with '$'. This
+         * supports $unset on legacy fields.
+         */
+        Status isUpdatableLegacy(const FieldRef& field);
+
+        /**
+         * Returns true, the position 'pos' of the first $-sign if present in 'fieldRef', and
+         * how many other $-signs were found in 'count'. Otherwise return false.
+         *
+         * Note:
+         *   isPositional assumes that the field is updatable. Call isUpdatable() above to
+         *   verify.
+         */
+        bool isPositional(const FieldRef& fieldRef, size_t* pos, size_t* count = NULL);
+
+    } // namespace fieldchecker
+
+} // namespace mongo