pub fn k_core(graph: &Subgraph, k: usize) -> BTreeSet<String>Expand description
k-core decomposition: the maximal induced subgraph in which every node has
degree at least k (§5.4).
Treats the graph as undirected, summing in- and out-degree. Parallel edges count once each — a node held in by three edges to one neighbour has degree three, which is what makes this a multigraph core.