Multiple categories in an OPML entry

What to do if a listed RSS feed belongs to multiple categories?

[SebastianX]

For some reason I want to create one OPML file containing RSS feeds where most of the feeds belong to more than one category.

This is a sample entry:
<outline type="rss" text="RSS Feed Title" title="RSS Feed Title" description="RSS Feed Description" xmlUrl="http://www.somedomain.com/rss.php?no=4711" htmlUrl="http://www.somedomain.com/page.php?no=4711" category="/Cat/SubCat" language="en-us" />

I didn't find information on a 'categories' attribute with standard delimiters (doh! I know ... arrays/lists should not be used in normalized structures) nor a sample of multiple instances of category in sub-outlines or so.

Usually folks create a OPML file per category, but in my example this would result in unwanted duplicates.

Any ideas? TIA!