public async Task EquationAsync(string eqn)
        {
            var action = new Protos.CalculatorDuplexCallback.Types.Equation
            {
                Eqn = eqn,
            };

            var callback = new Protos.CalculatorDuplexCallback
            {
                Equation = action
            };

            await _stream.WriteAsync(callback);
        }
 private async Task InvokeEquationAsync(Protos.CalculatorDuplexCallback.Types.Equation callback)
 {
     await _callback.EquationAsync(callback.Eqn);
 }