--This behavior changes the planes menu item image on rollover and mouseUp and passes the mouseUp event to the castMember script.Each item has 4 states:
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--1) neutral: when it's not selected and not rolled over
--2)rolledOver: when it's not selected and rolled over
--3)hilited: when it's selected and rolled over
--4)selected: when it's selected and not rolled over
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--Corresponding castMembers are always located in this particular order in the internal cast.It makes it possible to switch between them depending on the item's state by manipulating their castNum. So,in order to change the item's image when you go from, say, neutral to rolledOver state, all you have to do is to increase the item's sprite castNum by 1.