fn len(&self) -> usize {
        match self {
            MergeState::Single(Some(b)) => b.len(),
            MergeState::Double(MergeVariant::InProgress(b1,b2,_)) => b1.len() + b2.len(),
            MergeState::Double(MergeVariant::Complete(Some((b, _)))) => b.len(),
            _ => 0,
        }
    }
The number of actual updates contained in the level.
differential-dataflow/src/trace/implementations/spine_fueled.rs