Measures can contain less notes
than the time signature says. At the very beginning this is usually
handled as an upbeat (even if the first measure is not tagged as
measure 0 or the ‘implicit’ attribute is missing).
Here, the first and third measure contain only two quarters instead of
four.MusicXML Part1G2A411quarterA411quarterA411quarterA411quarterA411quarterA411quarterA411quarterA411quarterA411quarterA411quarterA411quarterA411quarter