You can create a FenwickTree with new:

>>> import AtCoder.FenwickTree qualified as FT
>>> ft <- FT.new @_ @Int 4 -- [0, 0, 0, 0]
>>> FT.nFt ft
4

It can perform point add and range sum in \(O(\log n)\) time:

>>> FT.add ft 0 3          -- [3, 0, 0, 0]
>>> FT.sum ft 0 3
3
>>> FT.add ft 2 3          -- [3, 0, 3, 0]
>>> FT.sum ft 0 3
6

You can create a FenwickTree with initial values using build:

>>> ft <- FT.build @_ @Int $ VU.fromList [3, 0, 3, 0]
>>> FT.add ft 1 2          -- [3, 2, 3, 0]
>>> FT.sum ft 0 3
8