PDF toc (bookmark) depth

Is there an easy way (like a parameter:) to limit the depth of the PDF bookmarks generated by the fo stylesheets? 

Right now, it looks like bookmarks are generated at all section levels, regardless of how deep the "real" toc is set for. I'd like to limit them to match the toc level.

Looking at the mailing list archive and Bob's book, I don't see anything about this topic, but I figured I'd ask on the list before diving into the code:).

If it makes a difference, I'm using the 1.76.1ns version of the stylesheets with XEP.

Thanks,
Dick Hamilton
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