It’s so basic, you wonder why no one thought of it earlier. The recording industry has unveiled a system it says will make it easier for artists and record companies to be compensated for digital music purchased online. The system is called GRid (Global Release Identifier,) and it works much like a UPC code attached to each song, allowing the seller to track songs sold. All sides seem to be guardedly optimistic about the system, although privacy advocates worry that the GRid could be used to pursue consumers who buy a tagged song and then allow it to be traded on a song-swapping site.