I see what you mean, Steve, but we’ve tended to play a few bars of the melody in between verses of carols too this week, and would not really regard those as riffs. The guitar intro to the likes of Layla, Money for Nothing, etc are famous riffs, but are not fragments of the melody so much as a few enticing notes in the same key and rhythm. With some like The Last Time and the Byrds’ Mr Tambourine Man, the riff is based on the same chords as the melody, so arguably is a harmony line.