># let symbol =
>    let m = Hashtbl.create 1 in
>    fun string ->
>      try Hashtbl.find m string with Not_found ->
>      Hashtbl.add m string string;
>      string;;
>val symbol : '_a -> '_a = <fun>

Nice, but perhaps it should return a different type to avoid accidentally
comparing an interned string with a non-interned one?

We would need an accessor to extract the string from an abstract type but
I suppose that would be inlined.