more Pure theory content;

--- a/src/Pure/pure_thy.scala	Sat Aug 04 01:03:39 2018 +0200
+++ b/src/Pure/pure_thy.scala	Sat Aug 04 15:49:54 2018 +0200
@@ -9,6 +9,7 @@
 
 object Pure_Thy
 {
+  val DUMMY: String = "dummy"
   val FUN: String = "fun"
   val PROP: String = "prop"
   val ITSELF: String = "itself"