Function subdigraph 

pub fn subdigraph<N, E, Ix>(
    digraph: &DiGraph<N, E, Ix>,
    nodes: &HashSet<N>,
) -> DiGraph<N, E, Ix>
where
    N: Node + Clone + Debug,
    E: EdgeWeight + Clone,
    Ix: IndexType,

Extracts a subgraph from a directed graph