prefix and postfix

(* prefix *)
let prefix pre set
    let list = SS.elements set in
    let rec addPrefix list = match element with
      (e :: l) -> ( pre ^ e ) :: addPrefix (l) in
    SS.of_list (addPrefix list)
;;

(* postfix *)
let postfix post set
    let list = SS.elements set in
    let rec addPostfix list = match element with
      (e :: l) -> ( e ^ post ) :: addPostfix (l) in
    SS.of_list (addPostfix list)
;;