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I’ve contributed an article to this issue, explaining some of the differences between Flex 3 and Flex 4, particularly in reference to the parent/child relationships between containers and the controls they contain. This has been one of the problems we’ve had to solve in order to make the drag-and-drop designer in my company’s Flash Platform IDE work with both Flash 3 (’Halo’) and Flash 4 (’Spark’) controls. If you have any plans to use Flex 4, the article may save you some time and grief... 
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