Not rewriting URLs for existing content using Virtuoso rules based URL Rewriter

Question

I'm using Virtuoso's rule base URL Rewriter and I want to tell it not to rewrite a URL if a file actually exists at the specified URL. Using Apache and mod_rewrite the relevant RewriteConds would look like this:

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d

But I can't find an alternative for this in Virtuoso. Any ideas?