SkhcDb.MusicParticipantSourceOperations on music's participant
type of music's participant
init ~music_id ~artist_id role creates a new relation by associated a music with music_id an artist with artist_id with a role.
insert db participant inserts participant inside db.
update_artist db old_id new_id replaces old_id by new_id.
equal m1 m2 tests whether m1 equals m2. All fields are used for the test.