smus is geared for "classical" scores, not free-form performances.  so its
event durations are classical (whole note, dotted quarter rest, etc.).
SMUS can tie notes together to build a "note event" with an unusual event
duration. The set of useful classical durations is very small.  So SMUS
needs only a handful of bits to encode an event duration.  This is very
compact.  It's also very easy to display in Common Music Notation (CMN).