src/Pure/Thy/thy_resources.scala

     {
       require(remove.forall(name => !is_required(name)))
       copy(theories = theories -- remove)
     }
 
-    lazy val theories_graph: Graph[Document.Node.Name, Unit] =
+    lazy val theory_graph: Graph[Document.Node.Name, Unit] =
     {
       val entries =
         for ((name, theory) <- theories.toList)
         yield ((name, ()), theory.node_header.imports.map(_._1).filter(theories.isDefinedAt(_)))
       Graph.make(entries, symmetric = true)(Document.Node.Name.Ordering)

   def purge_theories(session: Session, nodes: Option[List[Document.Node.Name]])
     : (List[Document.Node.Name], List[Document.Node.Name]) =
   {
     state.change_result(st =>
       {
-        val graph = st.theories_graph
+        val graph = st.theory_graph
         val all_nodes = graph.topological_order
 
         val purge =
           (if (nodes.isEmpty) all_nodes else nodes.get.filter(graph.defined(_))).
             filterNot(st.is_required(_)).toSet