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diff --git a/docs/teaching/exercises/answers/Answer3d.java b/docs/teaching/exercises/answers/Answer3d.java
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+++ b/docs/teaching/exercises/answers/Answer3d.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 support.Log;
+
+import figures.*;
+
+aspect Answer3d {
+
+    boolean Point.inGroup = false;
+
+    before(Point p): execution(void Group.add(FigureElement)) && args(p) {
+	if (p.inGroup) {
+	    throw new IllegalStateException();
+	} else {
+	    p.inGroup = true;
+	}
+    }
+}