Sum elements in l according to pairs. For each (i, count) in pairs, replace l[i:i+count] with sum(l[i:i+count]).
- When period is a positive integer, read pairs cyclically.
- When rump = False do not append incomplete final sum.
Examples:
abjad> l = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
abjad> listtools.sum_slices_at(l, [(0, 2)], period = 4)
[1, 2, 3, 9, 6, 7, 17, 10]
abjad> l = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
abjad> listtools.sum_slices_at(l, [(0, 3)], period = 4)
[6, 3, 15, 7, 27]
abjad> l = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
abjad> listtools.sum_slices_at(l, [(0, 4)], period = 4)
[6, 22, 27]
When period is not None, indices in pairs must be less than period.