src/Pure/System/command_line.scala

     def unapplySeq(list: List[String]): Option[List[List[String]]] = Some(chunks(list))
   }
 
   var debug = false
 
-  def tool(body: => Unit)
+  def tool(body: => Unit): Unit =
   {
     val thread =
       Isabelle_Thread.fork(name = "command_line", inherit_locals = true) {
         val rc =
           try { body; 0 }