Re-assembled the topology subsystem.
diff --git a/core/api/src/main/java/org/onlab/onos/net/topology/TopologyGraph.java b/core/api/src/main/java/org/onlab/onos/net/topology/TopologyGraph.java
new file mode 100644
index 0000000..70139cd
--- /dev/null
+++ b/core/api/src/main/java/org/onlab/onos/net/topology/TopologyGraph.java
@@ -0,0 +1,10 @@
+package org.onlab.onos.net.topology;
+
+import org.onlab.graph.Graph;
+
+/**
+ * Represents an immutable topology graph.
+ */
+public interface TopologyGraph extends Graph<TopologyVertex, TopologyEdge> {
+
+}