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diff --git a/lock.ur b/lock.ur
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+(* Copyright 2015 the Massachusetts Institute of Technology
+
+Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+this file except in compliance with the License.  You may obtain a copy of the
+License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software distributed
+under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+CONDITIONS OF ANY KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations under the License. *)
+
+table lock : { Title : string,
+               Holder : client }
+  PRIMARY KEY Title
+
+fun take page_title =
+  new_holder <- self;
+  result <- tryDml (INSERT INTO lock (Title, Holder)
+                    VALUES ({[page_title]}, {[new_holder]}));
+  return (Option.isNone result)
+
+fun release page_title =
+  requestor <- self;
+  dml (DELETE FROM lock
+       WHERE Holder = {[requestor]})