src/Pure/Tools/mkroot.scala

     session_dir: Path = Path.current,
     session_parent: String = "",
     init_repos: Boolean = false,
     title: String = "",
     author: String = "",
-    progress: Progress = new Progress)
+    progress: Progress = new Progress): Unit =
   {
     Isabelle_System.make_directory(session_dir)
 
     val name = proper_string(session_name) getOrElse session_dir.absolute_file.getName
     val parent = proper_string(session_parent) getOrElse Isabelle_System.getenv("ISABELLE_LOGIC")