src/Pure/Thy/sessions.scala

           case Nil => None
           case entries => Some(name -> entries.map(_.info))
         })
 
     def session_chapters: List[(String, String)] =
-      build_topological_order.map(name => name -> apply(name).chapter)
+      imports_topological_order.map(name => name -> apply(name).chapter)
 
     override def toString: String =
       imports_graph.keys_iterator.mkString("Sessions.Structure(", ", ", ")")
   }