It seems that he did not manage to finish it before he died , and that it was finished by another musician called Franz Xaver SÃ 1\/4 ssmayr . We do not know exactly how much of the music is by Mozart , and how much SÃ 1\/4 ssmayr had to add . The task was then given to another composer , Franz Xaver SÃ 1\/4 ssmayr .