src/Pure/System/isabelle_system.scala

     bash("cp -a " + File.bash_path(dir1) + " " + File.bash_path(dir2)).check
 
 
   /* 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
     File.platform_file(path)
   }