actually export unit parser;

--- a/src/Pure/Isar/value_parse.ML	Wed Sep 30 22:25:50 2009 +0200
+++ b/src/Pure/Isar/value_parse.ML	Wed Sep 30 22:26:25 2009 +0200
@@ -9,6 +9,7 @@
   val comma: 'a parser -> 'a parser
   val equal: 'a parser -> 'a parser
   val parens: 'a parser -> 'a parser
+  val unit: unit parser
   val pair: 'a parser -> 'b parser -> ('a * 'b) parser
   val triple: 'a parser -> 'b parser -> 'c parser -> ('a * 'b * 'c) parser
   val list: 'a parser -> 'a list parser