Just to give you a quick response: look into MusicXML's concept of a "cursor". Parsing a MusicXML document requires you to keep an internal state of a "position", which increments for every note (well, careful, not every note -- not the ones that contain a "chord" subelement!) and can be explicitly moved via the "backup" and "forward" elements: https://w3c.github.io/musicxml/musicxml-reference/elements/f...

For music with multiple voices, this gets easy to mess up. It's also prone to fractional errors in music with tuplets, because sometimes software chooses to use MusicXML position numbers that aren't evenly divisible into the rhythms used in a particular piece of music. That can result in a situation where the MusicXML cursor gets to a state that doesn't actually align with any of the music.