src/Pure/Tools/build.scala

                   if (ancestor_results.isEmpty) {
                     SHA1.shasum_meta_info(SHA1.digest(Path.explode("$POLYML_EXE")))
                   }
                   else SHA1.flat_shasum(ancestor_results.map(_.output_heap))
 
-                val do_store =
-                  build_heap || Sessions.is_pure(session_name) ||
-                  build_context.is_inner(session_name)
-
+                val do_store = build_heap || build_context.build_heap(session_name)
                 val (current, output_heap) = {
                   store.try_open_database(session_name) match {
                     case Some(db) =>
                       using(db)(store.read_build(_, session_name)) match {
                         case Some(build) =>