Function sort 

pub fn sort<N, F>(nodes: &[N], deps: F) -> Result<Vec<N>, ToposortError<N>>
where
    N: Clone + Eq + Hash + Debug,
    F: Fn(&N) -> Vec<N>,

Orders nodes so that each follows the nodes deps says it depends on. The order among nodes the dependencies leave free is unspecified. A dependency outside the node set is ignored; a node depending on itself is a cycle.