Add test for add_edge() return value

src/models.rs

@@ -9,7 +9,7 @@ impl From<Vertex> for usize {
     }
 }
 
-#[derive(Copy, Clone, PartialEq, Eq)]
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub struct Incidence(usize);
 
 struct VertexIncidenceHeader {
@@ -136,6 +136,15 @@ mod tests {
         assert_eq!(graph.vertex_count(), 10, "unexpected vertex count");
     }
 
+    #[test]
+    fn add_edge() {
+        let mut graph = AppendGraph::new();
+        let v1 = graph.add_vertex();
+        let v2 = graph.add_vertex();
+        let e = graph.add_edge(v1, v2);
+        assert_ne!(graph.add_edge(v1, v2), e, "unexpected duplicate edge");
+    }
+
     #[test]
     fn edge_count_empty() {
         let graph = AppendGraph::new();