diff --git a/src/util/Event.ml b/src/util/Event.ml
new file mode 100644
index 00000000..ec08501d
--- /dev/null
+++ b/src/util/Event.ml
@@ -0,0 +1,38 @@
+
+type 'a small_set =
+  | S0
+  | S1 of 'a
+  | S2 of 'a * 'a
+  | S3 of 'a * 'a * 'a * 'a small_set
+
+type 'a t = {
+  mutable set: ('a -> unit) small_set;
+}
+
+let[@unroll 1] rec cons_ f set = match set with
+  | S0 -> S1 f
+  | S1 f2 -> S2(f,f2)
+  | S2(f2,f3) -> S3 (f,f2,f3, S0)
+  | S3(f1,f2,f3,tl) -> S3 (f,f1,f2,cons_ f3 tl)
+
+let on (e:_ t) f : unit =
+  let set' = cons_ f e.set in
+  e.set <- set'
+
+module Emitter = struct
+  type nonrec 'a t = 'a t
+
+  let rec fire_set_ set x =
+    match set with
+    | S0 -> ()
+    | S1 f -> f x
+    | S2 (f1,f2) -> f1 x; f2 x
+    | S3 (f1,f2,f3,tl) -> f1 x; f2 x; f3 x; fire_set_ tl x
+
+  let[@inline] fire e x = fire_set_ e.set x
+end
+
+let make () =
+  let e = {set=S0} in
+  e, e
+
diff --git a/src/util/Event.mli b/src/util/Event.mli
new file mode 100644
index 00000000..09e3a9cb
--- /dev/null
+++ b/src/util/Event.mli
@@ -0,0 +1,15 @@
+
+(** {1 Observer pattern} *)
+
+type 'a t
+
+val on : 'a t -> ('a -> unit) -> unit
+
+module Emitter : sig
+  type 'a t
+
+  val fire : 'a t -> 'a -> unit
+end
+
+val make : unit -> 'a t * 'a Emitter.t
+