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The task:
Given a list of numbers L, implement a method sum(i, j) which returns
the sum from the sublist L[i:j] (including i, excluding j).
For example, given L = [1, 2, 3, 4, 5], sum(1, 3) should return
sum([2, 3]), which is 5.
You can assume that you can do some pre-processing...