Bug 138562 Splite extendsexcpetion to seperate sub class
The suggestion is Parent invalid Cycle extends Different type extends Not top level extends
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/core/DesignElement.java
/**
	 * Generates a clone copy of this element. When a report element is cloned,
	 * the basic principle is just copying the property value into the clone,
	 * the other things, like container references, child list references,
	 * listener references will not be cloned; that is, the clone is isolated
	 * from the design tree until it is added into a target design tree.
	 * 
	 * <p>
	 * When inserting the cloned element into the design tree, user needs to
	 * care about the element name confliction; that is, the client needs to
	 * call the method
	 * <code>{@link ReportDesignHandle#rename( DesignElementHandle )}</code>
	 * to change the element names.
	 * 
	 * @return Object the cloned design element.
	 * @throws CloneNotSupportedException
	 *             if clone is not supported.
	 * 
	 */
	public Object clone( ) throws CloneNotSupportedException
	{
		DesignElement element = (DesignElement) super.clone( );
		element.container = null;
		element.listeners = null;
		element.derived = null;
		element.extendsRef = null;
		element.cachedDefn = null;
		element.handle = null;
		element.baseId = NO_BASE_ID;
		element.propValues = new HashMap( );