Function residual_capacity 

fn residual_capacity<G>(
    network: G,
    edge: G::EdgeRef,
    target_vertex: G::NodeId,
    flow: G::EdgeWeight,
) -> G::EdgeWeight
where
    G: NodeIndexable + IntoEdges,
    G::EdgeWeight: Sub<Output = G::EdgeWeight> + PositiveMeasure,

Returns the residual capacity of given edge.