[flexcoders] how can i get a columnIndex from an item in my TileList?

[blc187]

I have a TileList that displays images.
I'd like to be able to mouseover one of the images and get the index 
of the column it's in.
Is there any way to get a row or column index on MouseOver?

For context, I am displaying a custom tooltip on mouseover and I'd 
like to be able to tell if I'm mousing over the last image in the row 
so that I can prevent my tooltip from displaying offscreen.