A common library of attributes for instrument definitians.

Usually attributes act like tags in that they all get combined into one Attributes set. This makes sense in some cases, but not all. For example, +mute and +cresc can be usefully combined, you could imagine applying +mute to the entire melody and wanting that to combine with individual articulations within. But +pizz-right is an articulation on its own, and it doesn't really make sense to, e.g. put +right on melody by itself.

And of course since attributes are all mashed into one set, the information about which ones were applied together is lost. For example, I think it's impossible for the lilypond backend to figure how to notate +pizz+right inside of +pizz since right-hand pizz is applied as a per-note articulation, while plain pizz is a mode. But it's trivial if the attribute is +pizz-right instead.