public async Task<Maybe<IOrder>> SubmitSellOrder(string symbol, Account account, double shares) {
            await Task.Yield();

            var id = Guid.NewGuid();
            var slippage = MoneyUtil.NinetyPercentOf(new Money(shares * 10, Currency.FromCurrencyCode(CurrencyCode.USD)));
            var order = new TestOrder() {
                Id = id, 
                Slippage = slippage, 
                IsExecuted = false
            };

            sellOrders.Add(id, order);

            return Maybe<IOrder>.Some(order);
        }
        public async Task<Maybe<IOrder>> SubmitBuyOrder(string symbol, Account account, Money amount) {
            await Task.Yield();

            var id = Guid.NewGuid();
            var slippage = MoneyUtil.TenPercentOf(amount);
            var order = new TestOrder() {
                Id = id, 
                Slippage = slippage, 
                IsExecuted = false
            };

            buyOrders.Add(id, order);

            return Maybe<IOrder>.Some(order);
        }