public static void CutNonReachable <TVertex, TEdge>(this IMutableBidirectionalGraph <TVertex, TEdge> graph,
IEnumerable <TVertex> toKeep)
where TEdge : IEdge <TVertex>
{
var reachable = graph.FindReachable(toKeep);
graph.RemoveVertexIf(x => !reachable.Contains(x));
}