rearrange some stuff

set_ext.ml

@@ -0,0 +1,15 @@
+module type S = sig
+  include Set.S
+  val exists_opt: (elt -> 'a option) -> t -> 'a option
+end
+
+module Make(Elt: Set.OrderedType) = struct
+  include Set.Make(Elt)
+
+  let exists_opt (type a) (f: elt -> a option) set =
+    let exception Found of a in
+    let f' x = Option.iter (fun x -> raise (Found x)) (f x) in
+    match iter f' set with
+    | exception Found x -> Some x
+    | _                 -> None
+end