[plt-scheme] Find-files exception when encountering unreadable directory

Hi,

I have troubles using find-files in scheme/file (3.99) to traverse a
directory hierarchy. For some reason, some some mounted .dmg images on
OS X tend to have a .Trashes folders that isn't readable by the user,
and find-files yields an exception when it encounters the volume's
top-level .Trashes folder. I cannot get rid of these exceptions, no
matter what I put into the predicate for find-files.

Am I missing something obvious? Or does anyone have another find-files
function at hand? (I'm dealing with rather large directories, so it
should be iterative/use accumulators.)

Best,

Erich

P.S. No, this is not a homework assignment. I know it's a good excercise
to write my own, but I'm pretty busy at the moment.