--- src/check.ml
+++ src/check.ml
@@ -37,7 +37,7 @@
 
 	  	let of_labels labels = List.fold_left (fun lbs lb -> add (lb, None) lbs) empty labels
 	  	let occurs lb lbs = mem (lb, None) lbs
-        let pretty lbs = "{ " ^ (flatten_strings ", " (map fst (elements lbs))) ^ " }"
+        let pretty lbs = "{ " ^ (flatten_strings ", " (List.map fst (elements lbs))) ^ " }"
 	  end
 
 
