Add comments for AppendGraph and Graph add_incidence()

src/models/graph.rs

@@ -54,6 +54,8 @@ impl Graph {
         }
     }
 
+    // Adds a single incidence of an edge, which is composed by two such incidences, to the
+    // incidences arena, and returns its index.
     fn add_incidence(&mut self, v1: Vertex, v2: Vertex) -> Edge {
         let edge = self.incidences.insert(IncidenceEntry {
             next: self.vertices[v1].first_incidence.take(),