Why this imp is accumulated instead of assigned?

[afboteros]

Modelling-and-Analysis-of-a-Vehicle-Routing-Problem-with-Time-Windows-in-Freight-Delivery/code/3. Test_GVNS_S1.py

Line 163 in 59b66df

Best_Imp += Imp

Hi Dung, would like to know if possible, why in the or_opt_search and two_opt_search, the best_imp is accumulated on every iteration instead of assigned (so replaced).

Thanks in advance.