Plus-separated categories

Somewhere I've viewed an RSS (if I remember correctly RSS 2.0) file
with single <category> tag but two categories separated with '+' sign.
Something like this:

<category>Music + Dreams</category>

Should my RSS aggregator which I currently write consider this as two
categories? What is this plus-separated format: standard, common
practice, or just a little mis-behavior of marginal users? Is it
necessary to take it into account?


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