src/Pure/Admin/build_spass.scala

 
   def build_spass(
     download_url: String = default_download_url,
     verbose: Boolean = false,
     progress: Progress = new Progress,
-    target_dir: Path = Path.current)
+    target_dir: Path = Path.current): Unit =
   {
     Isabelle_System.with_tmp_dir("build")(tmp_dir =>
     {
       Isabelle_System.require_command("bison", "flex")