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diff --git a/docs/teaching/exercises/answers/Answer2b.java b/docs/teaching/exercises/answers/Answer2b.java
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+++ b/docs/teaching/exercises/answers/Answer2b.java
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+/* *******************************************************************
+ * Copyright (c) 2002 Palo Alto Research Center, Incorporated (PARC).
+ * All rights reserved. 
+ * This program and the accompanying materials are made available 
+ * under the terms of the Common Public License v1.0 
+ * which accompanies this distribution and is available at 
+ * http://www.eclipse.org/legal/cpl-v10.html 
+ *  
+ * Contributors: 
+ *     PARC     initial implementation 
+ * ******************************************************************/
+
+package answers;
+
+import figures.Group;
+import figures.FigureElement;
+
+public aspect Answer2b {
+    before(FigureElement newValue):
+            call(void Group.add(FigureElement))
+            && args(newValue) {
+	if (newValue == null) {
+            throw new IllegalArgumentException("null not allowed");
+        }
+    }
+}