How to update express with CalculationEngine

[dboydboy]

I used CalculationEngine to manage and calculate a group of express，like as：

var engine = new CalculationEngine();
var context = new ExpressionContext();
engine.Add("x", "2", context);
engine.Add("y", "sqrt(x)", context);
engine.Add("z", "y * 3", context);

And i want to update the "x", and it could update the referenced express auto...
I tried the following:

engine.Remove("x");
engine.Add("x", "9", context);
engine.GetResult("y")

there is a exception like as:
System.ArgumentException:“No expression is associated with the name 'y'”
So, How could i update the "x"?