UnderlayingGraph: make graphs immutable

graph_tools/parameters.py

@@ -117,7 +117,7 @@ def _init_():
     def join_edges(self, b, e1, e2):
       g = b.value.copy(immutable=False)
       g.add_edge(b.boundary[e1], b.boundary[e2])
-      yield Boundary(b.boundary, g)
+      yield Boundary(b.boundary, g.copy(immutable=True))
 
     def project_and_canonize(self, sel, b):
       return Boundary(select(sel, b.boundary), b.value)