      \fitem{make_label_duration}() \emph{or} (start:numeric) \emph{or}
      (start:numeric, stop:numeric): %%
      returns a map associating to the event of a label, the duration of
      this event.  Events with no label do not appear in the map. 
      %%
      Called with no arguments, the events considered are all
      the events in the score. With \code{start}, only the events whose
      position is greater than \code{start} are considered. If a
      \code{stop} is specified, all events must have a position between
      \code{start} and \code{stop}.

      %%% -------------------------------------------------------

