type storage = {
  challenge : string;
}

type param = {
  new_challenge : string;
  attempt       : string;
}

let attempt (p: param) storage =
  (* if p.attempt <> storage.challenge then failwith "Failed challenge" else *)
  let contract : unit contract =
    Operation.get_contract sender in
  let transfer : operation =
    Operation.transaction (unit , contract , 10.00tz) in
  (* TODO: no syntax for functional updates yet *)
  (* let storage : storage = { storage with challenge = p.new_challenge } in *)
  (* for now, rebuild the record by hand. *)
  let storage : storage = { challenge = p.new_challenge }
  in ([] : operation list), storage