A FORM smus contains a required property "shdr" followed by any number of
parallel "track" data chunks "trak".  optional property chunks such as
"NAME", copyright "(c) ", and instrument reference "INS1" may also appear.
Any of the properties may be shared over a list of forms smus by putting
them in a prop smus.  [see the iff reference.]

 required property shdr 
 optional text chunks 
 optional property ins1 
 obsolete property inst 
 data chunk trak 
 note and rest events 
 set instrument sevent 
 set timesignature sevent 
 key signature sevent 
 dynamic mark sevent 
 set midi channel sevent 
 set midi preset sevent 
 instant music private sevents 
 end-mark sevents 
 more sevents to be defined