-- example input: [1,6,2,5,4]
-- result: [(1, 0), (6, 1), (2, 7), (5, 9), (4, 14)]
-- every element of the result list is a pair that has
-- the element of the list as the first element, and
-- as the second element the sum of all the preceding
-- elements.

before xs = inner xs 0
     where inner [] _ = []
           inner (x:[]) acc = [(x, acc)]
           inner (x:xs) acc = (x, acc):(inner xs (acc + x))