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
* @return the mask of the given property
	 */
	public String getPropertyMask( Module module, String propName )
	{
		DesignElement e = this;
		do
		{
			ArrayList masks = (ArrayList) e.getLocalProperty( module,
					DesignElement.PROPERTY_MASKS_PROP );
			if ( masks != null )
			{
				for ( int i = 0; i < masks.size( ); i++ )
				{
					PropertyMask mask = (PropertyMask) masks.get( i );
					if ( propName.equals( mask.getName( ) ) )
						return mask.getMask( );
				}
			}
			e = e.getExtendsElement( );
		} while ( e != null );
		return null;
	}
	/**
	 * Implements to cache a back-pointer from a referenced element. This
	 * element has an element reference property that can point to another
	 * "referencable" element. To maintain semantic consistency, the referenced
	 * element maintains a list of "clients" that identifies the elements that
	 * refer to it. The client list is used when the target element changes
	 * names or is deleted. In these cases, the change automatically updates the
	 * clients as well.