Ressurect seq attribute accidently removed

--- a/src/Pure/proof_general.ML	Wed Jul 20 07:40:23 2005 +0200
+++ b/src/Pure/proof_general.ML	Wed Jul 20 15:57:10 2005 +0200
@@ -176,6 +176,7 @@
       "pgip"
       ([("tag",    pgip_tag),
         ("class",  pgip_class),
+	("seq",    string_of_int (pgip_serial())),
         ("id",     !pgip_id)] @
        if_none (Option.map (single o (pair "destid")) (! pgip_refid)) [] @
        (* destid=refid since Isabelle only communicates back to sender,