+ /**
+ * For each vertex (node) already in the graph,
+ * make sure it also has a loopback link; this loopback
+ * can potentially already be in the graph, and in that
+ * case nothing will be done.
+ *
+ * If no loopback is specified for a node, we will use
+ * the loopback that is provided by the routing platform.
+ *
+ * After this function returns, any node in the graph
+ * will have a loopback attached to it.
+ */