It is commonly known that we can use Fenwick Tree to perform 2 tasks :

1) calculate the prefix sum (a[1]+a[2]+...+a[idx-1]+a[idx]) up to some index

2) increment a certain index by a certain value

but, my question is how can we modify the fenwick tree implementation so that instead of calculating prefix sums, we are calculating suffix sums so the sum we are now querying is (a[idx]+a[idx+1]+...+a[n-1]+a[n]), so I tried to change the implementation by basically swapping the indices that are accessed in the update and the query functions and I tried submitting in different problems and the submissions are accepted and I wanted to know if someone had a proof of why does it work ?