Function expand_closure 

fn expand_closure(
    output: CompactionOutput,
    seeds: &mut [Option<CompactionOutput>],
    seed_by_file: &HashMap<RegionFileId, usize>,
    remaining_files: &mut FileOverlapIndex<'_>,
) -> CompactionOutput

Expands one seed into its full merge closure: absorb every connected seed and every overlapping snapshot file, transitively. Growing output.inputs while iterating is what makes the expansion transitive.