diff -r 9dca3df78b6a -r d882abae3379 src/Pure/Proof/proof_syntax.ML
--- a/src/Pure/Proof/proof_syntax.ML	Mon Sep 20 15:27:00 2021 +0200
+++ b/src/Pure/Proof/proof_syntax.ML	Mon Sep 20 20:22:32 2021 +0200
@@ -46,7 +46,7 @@
   |> Sign.add_nonterminals_global
     [Binding.make ("param", \<^here>),
      Binding.make ("params", \<^here>)]
-  |> Sign.add_syntax Syntax.mode_default
+  |> Sign.syntax true Syntax.mode_default
     [("_Lam", [paramsT, proofT] ---> proofT, mixfix ("(1\<^bold>\<lambda>_./ _)", [0, 3], 3)),
      ("_Lam0", [paramT, paramsT] ---> paramsT, mixfix ("_/ _", [1, 0], 0)),
      ("_Lam0", [idtT, paramsT] ---> paramsT, mixfix ("_/ _", [1, 0], 0)),