More partial sums

Partial sums with unique input elements, for an alphabet of k elements, the time it takes to find a solution is at most 2^k, regardless of the size of the input. To flip that around, partial sums is solvable in O(n^k) where n is the size of the input and k is the size of the alphabet. Not that this does you any good, of course.

Notation duly abused.