diff -r b6a224676c04 -r 99dd8b4ef3fe src/Pure/PIDE/document_id.scala
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/Pure/PIDE/document_id.scala	Fri Jul 05 15:38:03 2013 +0200
@@ -0,0 +1,24 @@
+/*  Title:      Pure/PIDE/document_id.scala
+    Author:     Makarius
+
+Unique identifiers for document structure.
+
+NB: ML ticks forwards > 0, JVM ticks backwards < 0.
+*/
+
+package isabelle
+
+
+object Document_ID
+{
+  type ID = Long
+  val ID = Properties.Value.Long
+
+  type Version = ID
+  type Command = ID
+  type Exec = ID
+
+  val none: ID = 0
+  val make = Counter()
+}
+