Track the relevant span in the event vector using a conventional left-closed, right-open interval. Adjust
binsearch_scm_vector() to mimic
std::lower_bound. Ignore grace notes leading into unquoted music by choosing an appropriate threshold moment rather than choosing a later moment and walking back.
Check the result of
unsmob<Moment> before dereferencing and issue a programming_error if a non-Moment is found in the event vector.
process() before processing is due for the wrapped music.