Currently upbeat is only partially supported. The SimpleArrangementEngine supports shifting the beginning and/or end of an ActivityVector's activity segment forward or backward by a given number of ticks (by using the startShift and stopShift tags). This allows for simulating upbeat. Note, however, that upbeat at the beginning of the song (where it is usually most noticable and most useful) is not supported. An XML example that uses upbeat in the described fashion is SoundHelix-Popcorn.xml (included since version 0.8).