--- a/util/promise.lua	Thu Oct 25 14:17:32 2018 +0100
+++ b/util/promise.lua	Thu Oct 25 14:19:27 2018 +0100
@@ -147,4 +147,5 @@
 	all = all;
 	race = race;
 	try = try;
+	is_promise = is_promise;
 }