slightly off topic, but I think a beat or melody CAN be generally interchangeable if it's done right and with interchangeability in mind. See here: http://micropledge.com/projects/music-mash
Yeah I don't disagree despite what I wrote. Perhaps it's better put that interchangeability is a very powerful thing in software, but its more of a gimmick in music where no component takes nearly as long to describe and making parts that work independently is not seen as a good thing. The chords and tonal systems ensure interoperability anyway.