diff --git a/src/models.rs b/src/models.rs
index 3576374..d0ca964 100644
--- a/src/models.rs
+++ b/src/models.rs
@@ -154,6 +154,17 @@ mod tests {
         }
     }
 
+    #[test]
+    fn are_adjacent_vertex_self() {
+        let mut graph = Graph::new();
+        let v = graph.add_vertex();
+        assert_eq!(
+            graph.are_adjacent(v, v),
+            false,
+            "should not be adjacent to itself"
+        );
+    }
+
     #[test]
     fn are_adjacent_single_edge() {
         let mut graph = Graph::new();
@@ -269,6 +280,75 @@ mod tests {
         }
     }
 
+    #[test]
+    fn loop_edge() {
+        let mut graph = Graph::new();
+        let v = graph.add_vertex();
+        graph.add_edge(v, v);
+        assert_eq!(graph.vertex_count(), 1, "unexpected vertex count");
+        assert_eq!(graph.edge_count(), 1, "unexpected edge count");
+        assert_eq!(graph.degree(v), 2, "unexpected degree");
+        assert_eq!(
+            graph.are_adjacent(v, v),
+            true,
+            "vertex with loop edge should be adjacent to itself"
+        );
+        let mut neighbors = graph.neighbors(v);
+        assert_eq!(
+            neighbors.next(),
+            Some(v),
+            "vertex should be neighbor of itself"
+        );
+        assert_eq!(
+            neighbors.next(),
+            Some(v),
+            "vertex should be neighbor of itself twice"
+        );
+        assert_eq!(neighbors.next(), None, "two many neighbors from iterator");
+    }
+
+    #[test]
+    fn multiple_edges() {
+        let mult = 3;
+        let mut graph = Graph::new();
+        let vertices = [graph.add_vertex(), graph.add_vertex()];
+        for _ in 0..mult {
+            graph.add_edge(vertices[0], vertices[1]);
+        }
+        assert_eq!(
+            graph.vertex_count(),
+            vertices.len(),
+            "unexpected vertex count"
+        );
+        assert_eq!(graph.edge_count(), mult, "unexpected edge count");
+        for v in vertices {
+            assert_eq!(graph.degree(v), mult, "unexpected degree of {v:?}");
+        }
+        assert_eq!(
+            graph.are_adjacent(vertices[0], vertices[1]),
+            true,
+            "should be adjacent"
+        );
+        for i in 0..2 {
+            let mut neighbors = graph.neighbors(vertices[i]);
+            for j in 0..mult {
+                assert_eq!(
+                    neighbors.next(),
+                    Some(vertices[1 - i]),
+                    "neighbor {j} of vertex {:?} should be {:?}",
+                    vertices[i],
+                    vertices[1 - i]
+                );
+            }
+            assert_eq!(
+                neighbors.next(),
+                None,
+                "two many neighbors of {:?} from iterator",
+                vertices[i]
+            );
+        }
+    }
+
     fn make_test_graph() -> (Graph, [Vertex; 10]) {
         let mut graph = Graph::new();
         let vertices: [Vertex; 10] = core::array::from_fn(|_| graph.add_vertex());