public Functions(Cobra tradingSystem)
        {
            if (tradingSystem == null)
            {
                throw new ArgumentNullException(nameof(tradingSystem));
            }

            this.tradingSystem = tradingSystem;
        }
        public Functions(Cobra tradingSystem, IRateProvider rateProvider)
        {
            if (tradingSystem == null)
            {
                throw new ArgumentNullException("system");
            }

            this.tradingSystem = tradingSystem;
            this.rateProvider = rateProvider;
        }