Add todo

src/models.rs

@@ -1,3 +1,4 @@
+// TODO: Vertex (and Incidence?) should be references of Graph data, not plain indices into the internal (Vec) data structures.
 // TODO: Vertex value must not be public.
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub struct Vertex(pub usize);