diff -r 5eb932e604a2 -r eab6ce8368fa src/HOL/Library/Countable_Set.thy
--- a/src/HOL/Library/Countable_Set.thy	Wed Jan 10 15:21:49 2018 +0100
+++ b/src/HOL/Library/Countable_Set.thy	Wed Jan 10 15:25:09 2018 +0100
@@ -342,7 +342,7 @@
 qed
 
 lemma transfer_countable[transfer_rule]:
-  "bi_unique R \<Longrightarrow> rel_fun (rel_set R) op = countable countable"
+  "bi_unique R \<Longrightarrow> rel_fun (rel_set R) (=) countable countable"
   by (rule rel_funI, erule (1) bi_unique_rel_set_lemma)
      (auto dest: countable_image_inj_on)