diff -r 0b56d9c90dcf -r 2b3f02227c35 src/Pure/Syntax/syn_ext.ML
--- a/src/Pure/Syntax/syn_ext.ML	Sat Sep 01 00:20:22 2001 +0200
+++ b/src/Pure/Syntax/syn_ext.ML	Sat Sep 01 00:20:44 2001 +0200
@@ -311,7 +311,7 @@
     SynExt {
       logtypes = logtypes',
       xprods = xprods,
-      consts = consts union mfix_consts,
+      consts = consts union_string mfix_consts,
       prmodes = distinct (map (fn (m, _, _) => m) tokentrfuns),
       parse_ast_translation = parse_ast_translation,
       parse_rules = parse_rules,