src/Pure/System/isabelle_system.scala

   def download_file(url_name: String, file: Path, progress: Progress = new Progress): Unit =
     Bytes.write(file, download(url_name, progress = progress).bytes)
 
   object Download extends Scala.Fun("download", thread = true) {
     val here = Scala_Project.here
-    override def invoke(args: List[Bytes]): List[Bytes] =
+    override def invoke(session: Session, args: List[Bytes]): List[Bytes] =
       args.map(url => download(url.text).bytes)
   }
 
 
   /* repositories */