src/Pure/PIDE/headless.scala

       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)
+        Graph.make(entries, symmetric = true)(Document.Node.Name.Theory_Ordering)
       }
 
       def is_required(name: Document.Node.Name): Boolean = required.isDefinedAt(name)
 
       def insert_required(id: UUID.T, names: List[Document.Node.Name]): State =