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/extension/ModelExtensibilityProvider.java
{
	
	/**
	 * The name of the extension which is used to extend the extendable element.
	 */
	String extensionName = null;
	/**
	 * Constructs this provider with the element to extend, and extension name.
	 * 
	 * @param element the element to extend
	 * @param extensionName the extension name
	 */
	
	public ModelExtensibilityProvider( DesignElement element, String extensionName )
	{
		super( element );
		
		this.extensionName = extensionName;
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.birt.report.model.extension.IExtendableElement#getExtDefn()
	 */
	public ExtensionElementDefn getExtDefn( )
	{
		if ( extensionName == null )