--- a/src/HOL/Statespace/DistinctTreeProver.thy	Sun Jan 06 13:44:33 2019 +0100
+++ b/src/HOL/Statespace/DistinctTreeProver.thy	Sun Jan 06 15:04:34 2019 +0100
@@ -630,6 +630,6 @@
 text \<open>Now we have all the theorems in place that are needed for the
 certificate generating ML functions.\<close>
 
-ML_file "distinct_tree_prover.ML"
+ML_file \<open>distinct_tree_prover.ML\<close>
 
 end