It also affects the digital markup of music. Music can't be expressed with a tree structure as there is lots of overlap. A simple example is a slur. It crosses

It also affects the digital markup of music. Music can't be expressed with a tree structure as there is lots of overlap. A simple example is a slur. It crosses bar lines. <bar1> ... <slur> ... </bar1> ... </slur> <bar2> uhoh! In fact there are many mutually incompatible hierarchies in music engraving. It's funny though MusicXML is a popular interchange format, but internally looks nothing like what one would naively expect.