src/Pure/System/process_result.scala

 */
 
 package isabelle
 
 final case class Process_Result(
-  out_lines: List[String], err_lines: List[String], rc: Int, timeout: Option[Time])
+  rc: Int,
+  out_lines: List[String] = Nil,
+  err_lines: List[String] = Nil,
+  timeout: Option[Time] = None)
 {
   def out: String = cat_lines(out_lines)
   def err: String = cat_lines(err_lines)
 
   def error(s: String, err_rc: Int = 0): Process_Result =

   def check: Process_Result =
     if (ok) this
     else if (interrupted) throw Exn.Interrupt()
     else Library.error(err)
 
+  def clear: Process_Result = copy(out_lines = Nil, err_lines = Nil)
+
   def print: Process_Result =
   {
     Output.warning(Library.trim_line(err))
     Output.writeln(Library.trim_line(out))
-    copy(out_lines = Nil, err_lines = Nil)
+    clear
   }
 }