Returns the number of active beats in a rhythm string of an instrument in def-section-timesheet. The value can be used to determine the trim length so that the number of elements fit perfectly the rhythmic string.
(def-instrument-timesheet ...
...
piano (pick-random-init rhythm-source2 0.332512)
(symbol-trim (eventlen) (gen-random-successive 0.167 16 '(a b c d e -b -c))) with '(64)