Function find_join 

fn find_join<G, F>(
    graph: &G,
    edge: G::EdgeRef,
    mate: &[Option<G::NodeId>],
    label: &mut [Label<G>],
    first_inner: &mut [usize],
    visitor: F,
)
where
    G: IntoEdges + NodeIndexable + Visitable,
    F: FnMut(G::NodeId),