+class Edge<Tool t1, Tool t2> : EdgeBase<t1, t2, (empty)>;
+
+// Edge and SimpleEdge are synonyms.
+class SimpleEdge<Tool t1, Tool t2> : EdgeBase<t1, t2, (empty)>;
+
+// Optionally enabled edge.
+class OptionalEdge<Tool t1, Tool t2, dag props> : EdgeBase<t1, t2, props>;
+
+class CompilationGraph<list<EdgeBase> lst> {
+ list<EdgeBase> edges = lst;