public JsonResult Refund(IEnumerable <CoinViewModel> deposit)
        {
            Contract.Requires(deposit != null);

            var depositCoins = deposit
                               .Select(x => new Coin(x.ParValue, x.Count))
                               .ToArray();
            var command = new RefundCommand(depositCoins);
            var @event  = _refundCommandHandler.Execute(command);

            return(Json(@event));
        }