Function merge_pair 

fn merge_pair(
    adjacency: &mut [BTreeMap<u32, Interface>],
    uf: &mut UnionFind,
    a: u32,
    b: u32,
    iface: Interface,
    heap: &mut BinaryHeap<Candidate>,
)

Merge regions a and b at the offset their interface implies, fold their adjacency together, and queue the interfaces that changed.