How to realize coverage comparison?

[QuXing9]

Firstly, Congratulations on your work, it is excellent!

And I need your help with a problem after reading the paper carefully. As you mentioned in the Grammar-Aware Trimming Strategy section of your article :

otherwise, it attempts to trim a subtree n from tree (Line 6–7). If the coverage is different after n is trimmed, then n cannot be trimmed (Line 12–14)

I wonder how to compare the coverage changes before and after trimming the test case. Which piece of code implements this functionality? I cannot find the code because of my poor programming ability.

Looking forward to your reply, Thanks in advance!