public void Execute_DerivativeNonExistentFunction_ErrorReturned()
        {
            var          fs             = new FunctionStorage();
            var          command        = new DerivativeOfTheFunction("Name");
            const string expectedResult = "Name не найдена";

            Assert.AreEqual(expectedResult, command.Execute(fs));
        }
        public void Execute_DerivativeExistingFunction_DerivativeWillBeCalculated()
        {
            var fs = new FunctionStorage();

            fs.Add("Name", new Linear(2, 1));
            var          command        = new DerivativeOfTheFunction("Name");
            var          res            = command.Execute(fs);
            const string expectedResult = "Функция добавлена в хранилище.\nПроизводная Name = 2";

            Assert.AreEqual(expectedResult, res);
        }