src/Pure/Thy/sessions.scala

--- a/src/Pure/Thy/sessions.scala	Sun Apr 23 22:00:15 2017 +0200
+++ b/src/Pure/Thy/sessions.scala	Sun Apr 23 23:06:50 2017 +0200
@@ -448,6 +448,9 @@
     def build_topological_order: List[Info] =
       build_graph.topological_order.map(apply(_))
 
+    def imports_ancestors(name: String): List[String] =
+      imports_graph.all_preds(List(name)).tail.reverse
+
     def imports_topological_order: List[Info] =
       imports_graph.topological_order.map(apply(_))