src/Pure/PIDE/prover.scala

 
   def protocol_command_raw(name: String, args: List[Bytes]): Unit =
     command_input match {
       case Some(thread) if thread.is_active =>
         if (trace) {
-          val payload = (0 /: args)({ case (n, b) => n + b.length })
+          val payload = args.foldLeft(0) { case (n, b) => n + b.length }
           Output.writeln(
             "protocol_command " + name + ", args = " + args.length + ", payload = " + payload)
         }
         thread.send(Bytes(name) :: args)
       case _ => error("Inactive prover input thread for command " + quote(name))