Function validate_topo_sort 

pub fn validate_topo_sort<Id, PredsIter>(
    topo_sort: impl IntoIterator<Item = Id>,
    preds_fn: impl FnMut(Id) -> PredsIter,
) -> Result<(), (Id, Id)>
where
    Id: Copy + Eq + Ord,
    PredsIter: IntoIterator<Item = Id>,

Validates the topo sort.

Returns Ok(()) if it is valid. Returns Err((pred, succ)) where pred comes after succ when the sort is invalid. Panics if a node returned by the pred_fn is missing from the sort.