added id property;

--- a/src/Pure/General/markup.ML	Thu Jan 03 17:50:41 2008 +0100
+++ b/src/Pure/General/markup.ML	Thu Jan 03 17:50:42 2008 +0100
@@ -14,6 +14,7 @@
   val none: T
   val properties: (string * string) list -> T -> T
   val nameN: string
+  val idN: string
   val kindN: string
   val internalK: string
   val property_internal: property
@@ -95,6 +96,7 @@
 fun markup_int elem prop = (elem, fn i => (elem, [(prop, Int.toString i)]): T);
 
 val nameN = "name";
+val idN = "id";
 
 
 (* kind *)