src/HOL/Library/Preorder.thy

--- a/src/HOL/Library/Preorder.thy	Wed Jan 10 13:35:56 2018 +0100
+++ b/src/HOL/Library/Preorder.thy	Wed Jan 10 15:21:49 2018 +0100
@@ -13,7 +13,7 @@
   where "equiv x y \<longleftrightarrow> x \<le> y \<and> y \<le> x"
 
 notation
-  equiv ("op \<approx>") and
+  equiv ("'(\<approx>')") and
   equiv ("(_/ \<approx> _)"  [51, 51] 50)
 
 lemma refl [iff]: "x \<approx> x"