Function span_len 

fn span_len<B: SpineBatch>(span: &Span<B::Time, B>) -> usize

The number of updates in a span: its batch’s length, or zero when it carries none.

This is how the spine asks whether a span has updates. Span::has_updates answers the weaker question of whether a batch is present, which agrees with this only for producers that report absence when they built nothing; the spine can measure instead, and does.