Function collapse_stack_multilevel_with_hierarchy 

pub fn collapse_stack_multilevel_with_hierarchy<T>(
    stack: &mut SparseIoStack,
    proj_kn: &DMatrix<f32>,
    batch_membership: &[T],
    params: &MultilevelParams,
) -> Result<StackCollapseOut>
where
    T: Sync + Send + Hash + Eq + Clone + ToString,

The SparseIoStack collapse, also returning the per-level cell → pb membership (finest first) the layers share — the stack counterpart of collapse_columns_multilevel_with_hierarchy. With params.keep_finest_stats the finest level keeps its sufficient statistics, which CollapsedOut::observed_counts reads.