gnome_canvas_get_item_at() and transparency

[9kGemusen <9kGemusen myrealbox com>]

I would like to have one transparent item cover a set of non-transparent
items. This would give me a way to have the set of objects underneath
the transparent item act as one. I run into problems, though, when I
call gnome_canvas_get_item_at(), expecting to get the transparent layer.
It seems to ignore transparent items. Is there a way to recognize a
position on the logical layer? Or a better way to implement this?

Many thanks, please CC,
Gemusen