Time Fractional FreeFem++ Code of Heat equation

Hello, dear all, i am trying time fractional of Heat equation in FreeFem++. I am very beginner to this topic.
Here, is my code>
Heat_Frac.edp (2.8 KB)

Caputo fractional derivative of order \alpha = \frac{1}{2}
image

Optimized Error is coming in my code. I don’t know why. Let me know please if anyone get any errors in it.
Thanks in advance.