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
Object value = propValues.get( prop.getName( ) );
		assert value == null || value instanceof StructRefValue;
		if ( value == null || module == null )
			return (StructRefValue) value;
		StructRefValue ref = (StructRefValue) value;
		if ( ref.isResolved( ) )
			return ref;
		
		
		
		
		
		
		StructRefPropertyType refType = (StructRefPropertyType) prop.getType( );
		refType.resolve( module, prop, ref );
		if ( ref.isResolved( ) )
			ref.getTargetStructure( ).addClient( this, prop.getName( ) );
		return ref;
	}
	/**
	 * Returns the list of elements that extend this one.
	 * 
	 * @return the list of elements. The list is always non-null.
	 */
	public List getDerived( )
	{
		if ( derived != null )
			return new ArrayList( derived );
		return new ArrayList( );
	}