Should have reduced memory footprint of MvS purge operation

state/modelverse_state/main.py

@@ -426,7 +426,8 @@ class ModelverseState(object):
 
     def purge(self):
         self.garbage_collect()
-        values = set(self.nodes) | set(self.edges)
+        values = set(self.edges)
+        values.update(self.nodes)
         visit_list = [self.root]
 
         while visit_list: