Function as_timespans 

pub fn as_timespans<'a>(
    stream: &'a Stream,
    flatten: Flatten,
    filter: Option<fn(&StreamElement) -> bool>,
) -> TimespanTree<'a>

Every element of a stream, nested ones included, as a timespan: music21’s asTimespans. filter keeps only the elements it accepts, as music21’s classList does; the streams are walked into either way.