diff -r 1005c50b2750 -r 391ea80ff27c src/Pure/System/isabelle_system.scala
--- a/src/Pure/System/isabelle_system.scala	Wed Mar 18 17:44:27 2020 +0100
+++ b/src/Pure/System/isabelle_system.scala	Wed Mar 18 18:06:36 2020 +0100
@@ -170,7 +170,7 @@
 
   /* tmp files */
 
-  private def isabelle_tmp_prefix(): JFile =
+  def isabelle_tmp_prefix(): JFile =
   {
     val path = Path.explode("$ISABELLE_TMP_PREFIX")
     path.file.mkdirs  // low-level mkdirs