Function dfs 

fn dfs<G: NodeIndexable + IntoNeighborsDirected + IntoNodeIdentifiers + Visitable>(
    graph: G,
    start: G::NodeId,
    order_map: &OrderMap<G::NodeId>,
    valid_order: impl FnMut(TopologicalPosition) -> Result<bool, Cycle<G::NodeId>>,
    res: &mut BTreeMap<TopologicalPosition, G::NodeId>,
    discovered: &mut FixedBitSet,
    finished: &mut FixedBitSet,
) -> Result<(), Cycle<G::NodeId>>
where
    G::NodeId: IndexType,

Traverse nodes in graph in DFS order, starting from start, for as long as the predicate valid_order returns true on the current node’s order.