Function stratify 

pub fn stratify<T, R, F>(
    graph: &Graph,
    source: &Node<T>,
    rules: Vec<StratifyRule<R>>,
    classifier: F,
    opts: StratifyOptions,
) -> Stratified<T, R>
where
    T: Clone + 'static,
    R: Clone + 'static,
    F: Fn(&R, &T) -> bool + 'static,

Creates or computes stratify.