diff --git a/src/pl/plpgsql/src/gram.y b/src/pl/plpgsql/src/gram.y
index 4b2dce8cf0758ca05fc59c7a345c3d1a9c031085..4967a2a6a79eef7509790989fb3c7537a72750cc 100644
--- a/src/pl/plpgsql/src/gram.y
+++ b/src/pl/plpgsql/src/gram.y
@@ -746,7 +746,7 @@ decl_defkey		: assign_operator
 				| K_DEFAULT
 				;
 
-assign_operator	: '='
+assign_operator	: '='	/* not documented because it might be removed someday */
 				| COLON_EQUALS
 				;