Opposite of Tk::Canvas::canvasx()?

Does anybody know what is the exact calculation that:

  $x2 = $canvas->canvasx($x1);

does? I need the ability to reverse it, ie. get $x1,
given $x2. I couldn't find a built-in function to do
it, and I couldn't reverse engineer it either. So, I
snooped around the source, but I couldn't even find
where that function is defined!

Any ideas?
Thanks,
--Ala


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