diff --git a/lib/identifier.ml b/lib/identifier.ml
index 50e5bfc..fb94397 100644
--- a/lib/identifier.ml
+++ b/lib/identifier.ml
@@ -1,5 +1,6 @@
 type t = string [@@deriving eq, ord, show]
 
+(** Generate new unique id *)
 let fresh =
   let r = ref 0 in
   fun () ->