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diff --git a/server/src/com/vaadin/terminal/Extension.java b/server/src/com/vaadin/terminal/Extension.java
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+/*
+ * Copyright 2011 Vaadin Ltd.
+ * 
+ * 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.
+ */
+
+package com.vaadin.terminal;
+
+import com.vaadin.terminal.gwt.server.ClientConnector;
+
+/**
+ * An extension is an entity that is attached to a Component or another
+ * Extension and independently communicates between client and server.
+ * <p>
+ * An extension can only be attached once. It is not supported to move an
+ * extension from one target to another.
+ * <p>
+ * Extensions can use shared state and RPC in the same way as components.
+ * 
+ * @author Vaadin Ltd
+ * @since 7.0.0
+ */
+public interface Extension extends ClientConnector {
+    /*
+     * Currently just an empty marker interface to distinguish between
+     * extensions and other connectors, e.g. components
+     */
+}