src/Pure/Tools/isabelle_process.scala

     val PRIORITY = Value("PRIORITY")
     val TRACING = Value("TRACING")
     val WARNING = Value("WARNING")
     val ERROR = Value("ERROR")
     val DEBUG = Value("DEBUG")
-    val PROMPT = Value("PROMPT")
     // messages codes
     val code = Map(
       ('A' : Int) -> Kind.INIT,
       ('B' : Int) -> Kind.STATUS,
       ('C' : Int) -> Kind.WRITELN,
       ('D' : Int) -> Kind.PRIORITY,
       ('E' : Int) -> Kind.TRACING,
       ('F' : Int) -> Kind.WARNING,
       ('G' : Int) -> Kind.ERROR,
       ('H' : Int) -> Kind.DEBUG,
-      ('I' : Int) -> Kind.PROMPT,
       ('0' : Int) -> Kind.SYSTEM,
       ('1' : Int) -> Kind.STDIN,
       ('2' : Int) -> Kind.STDOUT,
       ('3' : Int) -> Kind.SIGNAL,
       ('4' : Int) -> Kind.EXIT)

     def is_system(kind: Value) =
       kind == SYSTEM ||
       kind == STDIN ||
       kind == SIGNAL ||
       kind == EXIT ||
-      kind == STATUS ||
-      kind == PROMPT
+      kind == STATUS
   }
 
   class Result(val kind: Kind.Value, val props: Properties, val result: String) {
     override def toString = {
       val res = XML.content(YXML.parse_failsafe(result)).mkString("")