Type casting IEnumerator to generic IEnumerator<>

cast.fs

open System
open System.Collections
open System.Collections.Generic

let cast<'a> : IEnumerator -> IEnumerator<'a> = function
   | :? IEnumerator<'a> as e -> e
   | e ->
      {
         new IEnumerator<'a> with
            member __.Current = e.Current :?> 'a
         interface IDisposable with
            member __.Dispose () =
               match e with
                  | :? IDisposable as d -> d.Dispose ()
                  | _ -> ()
         interface IEnumerator with
            member __.Current = e.Current
            member __.MoveNext () = e.MoveNext ()
            member __.Reset () = e.Reset ()
      }