diff -r c88d1c36c9c3 -r 1bd268ab885c src/Pure/Syntax/syntax.ML
--- a/src/Pure/Syntax/syntax.ML	Thu Jun 22 14:27:13 2017 +0200
+++ b/src/Pure/Syntax/syntax.ML	Thu Jun 22 15:20:32 2017 +0200
@@ -540,7 +540,7 @@
           else
             let
               val input' = new_xprods2 @ input1;
-              val gram' = Lazy.lazy (fn () => Parser.make_gram input');
+              val gram' = Lazy.lazy_name "Syntax.merge_syntax" (fn () => Parser.make_gram input');
             in (input', gram') end);
   in
     Syntax