1 files changed, 13 insertions, 0 deletions
diff --git a/graph/src/lib.rs b/graph/src/lib.rs
index 79f9646..26159c6 100644
--- a/graph/src/lib.rs
+++ b/graph/src/lib.rs
@@ -214,6 +214,19 @@ impl<T: Hash + Eq + PartialEq + Clone + Copy + Ord + PartialOrd + Display + Debu
 {
 }
 
+/// A parents-knowing graph can return an iterator of parents for any
+/// node.
+pub trait ParentsGraph: Graph {
+    /// The type of an iterator that goes through the parents of a
+    /// node.
+    type Iter<'a>: Iterator<Item = usize> + 'a
+    where
+        Self: 'a;
+
+    /// Return an iterator of parents for a node.
+    fn parents_of(&self, node_id: usize) -> Result<<Self as ParentsGraph>::Iter<'_>, Error>;
+}
+
 /// A labelled graph is just a graph with labels associated to
 /// vertices and / or edges.
 ///